Adds descending order sort to argsort

[sterrettm2]

This patch adds support for descending argsort and tests for it.

Comparing simdargsort.*random_1 to simd_revargsort.*random_1 (from ./benchexe)
Benchmark                                                                           Time             CPU      Time Old      Time New       CPU Old       CPU New
----------------------------------------------------------------------------------------------------------------------------------------------------------------
[simdargsort.*random_1 vs. simd_revargsort.*random_1]28/int64_t                  +0.0351         +0.0352           700           724           700           724
[simdargsort.*random_1 vs. simd_revargsort.*random_1]k/int64_t                   +0.0106         +0.0106         10495         10607         10495         10606
[simdargsort.*random_1 vs. simd_revargsort.*random_1]00k/int64_t                 +0.0127         +0.0128       1984814       2010041       1984600       2010030
[simdargsort.*random_1 vs. simd_revargsort.*random_1]m/int64_t                   +0.0125         +0.0127      35326504      35769670      35321177      35768358
[simdargsort.*random_1 vs. simd_revargsort.*random_1]0m/int64_t                  -0.0015         -0.0015    1008048326    1006544869    1007932916    1006469941
[simdargsort.*random_1 vs. simd_revargsort.*random_1]00m/int64_t                 -0.0008         -0.0007   18740949217   18725879386   18738472186   18724539794
[simdargsort.*random_1 vs. simd_revargsort.*random_1]28/uint64_t                 +0.0265         +0.0266           713           732           713           732
[simdargsort.*random_1 vs. simd_revargsort.*random_1]k/uint64_t                  +0.0110         +0.0111         10492         10607         10491         10607
[simdargsort.*random_1 vs. simd_revargsort.*random_1]00k/uint64_t                +0.0089         +0.0090       1984786       2002470       1984602       2002367
[simdargsort.*random_1 vs. simd_revargsort.*random_1]m/uint64_t                  +0.0142         +0.0144      35019575      35517267      35011788      35514938
[simdargsort.*random_1 vs. simd_revargsort.*random_1]0m/uint64_t                 +0.0014         +0.0015    1005610608    1006975763    1005453293    1006926654
[simdargsort.*random_1 vs. simd_revargsort.*random_1]00m/uint64_t                -0.0016         -0.0015   18755431656   18725763887   18753024082   18724198698
[simdargsort.*random_1 vs. simd_revargsort.*random_1]28/double                   +0.0341         +0.0341           599           620           599           620
[simdargsort.*random_1 vs. simd_revargsort.*random_1]k/double                    +0.0134         +0.0134          9352          9477          9351          9477
[simdargsort.*random_1 vs. simd_revargsort.*random_1]00k/double                  +0.0081         +0.0081       1936659       1952396       1936464       1952241
[simdargsort.*random_1 vs. simd_revargsort.*random_1]m/double                    +0.0138         +0.0138      34428106      34903675      34426293      34900722
[simdargsort.*random_1 vs. simd_revargsort.*random_1]0m/double                   -0.0006         -0.0005    1009933569    1009357736    1009741924    1009258251
[simdargsort.*random_1 vs. simd_revargsort.*random_1]00m/double                  -0.0010         -0.0009   18553309038   18535103083   18550992608   18533609826
[simdargsort.*random_1 vs. simd_revargsort.*random_1]28/int32_t                  +0.0138         +0.0138           621           629           621           629
[simdargsort.*random_1 vs. simd_revargsort.*random_1]k/int32_t                   +0.0173         +0.0173         10487         10668         10486         10668
[simdargsort.*random_1 vs. simd_revargsort.*random_1]00k/int32_t                 +0.0187         +0.0187       1715798       1747838       1715659       1747715
[simdargsort.*random_1 vs. simd_revargsort.*random_1]m/int32_t                   +0.0159         +0.0160      27752094      28193729      27747527      28191687
[simdargsort.*random_1 vs. simd_revargsort.*random_1]0m/int32_t                  +0.0003         +0.0004     813506313     813786134     813346264     813637191
[simdargsort.*random_1 vs. simd_revargsort.*random_1]00m/int32_t                 -0.0026         -0.0026   16372721091   16329909619   16370509091   16328529689
[simdargsort.*random_1 vs. simd_revargsort.*random_1]28/uint32_t                 +0.0167         +0.0167           620           631           620           631
[simdargsort.*random_1 vs. simd_revargsort.*random_1]k/uint32_t                  +0.0160         +0.0161         10499         10667         10498         10667
[simdargsort.*random_1 vs. simd_revargsort.*random_1]00k/uint32_t                +0.0081         +0.0081       1726447       1740387       1726247       1740274
[simdargsort.*random_1 vs. simd_revargsort.*random_1]m/uint32_t                  +0.0087         +0.0088      27792174      28034826      27789285      28034417
[simdargsort.*random_1 vs. simd_revargsort.*random_1]0m/uint32_t                 +0.0010         +0.0010     813359007     814143233     813226954     814051882
[simdargsort.*random_1 vs. simd_revargsort.*random_1]00m/uint32_t                -0.0024         -0.0024   16363564108   16323686834   16361610455   16322444405
[simdargsort.*random_1 vs. simd_revargsort.*random_1]28/float                    +0.0235         +0.0236           667           683           667           683
[simdargsort.*random_1 vs. simd_revargsort.*random_1]k/float                     +0.0138         +0.0138          9924         10060          9923         10060
[simdargsort.*random_1 vs. simd_revargsort.*random_1]00k/float                   +0.0121         +0.0122       1811155       1833046       1810910       1832930
[simdargsort.*random_1 vs. simd_revargsort.*random_1]m/float                     +0.0074         +0.0074      28442530      28653620      28439292      28649646
[simdargsort.*random_1 vs. simd_revargsort.*random_1]0m/float                    +0.0008         +0.0008     824497949     825123277     824381667     825026386
[simdargsort.*random_1 vs. simd_revargsort.*random_1]00m/float                   -0.0040         -0.0040   16435398683   16369311769   16433216995   16368255149
OVERALL_GEOMEAN                                                                  +0.0100         +0.0101             0             0             0             0

[r-devulap]

Looks good. Thanks @sterrettm2!