ci: Increase the benchmark rustc version to 2025-12-01

[tgross35]

Zerocopy (an indirect test dependency) has started requiring recently-stabilized features, so upgrade our benchmark toolchain to match.

ci: allow-regressions

[tgross35]

Most changes were within 1%. A few of the nicest improvements, LLVM must have gotten better at working with u16 since f16 ops account for a lot of them:

 icount::icount_bench_fdimf128_group::icount_bench_fdimf128 logspace:setup_fdimf128()
  Baselines:                      softfloat|softfloat (old)
  Instructions:                       89335|93504                (-4.45863%) [-1.04667x]
  L1 Hits:                           118819|125635               (-5.42524%) [-1.05736x]
  LL Hits:                                7|7                    (No change)
  RAM Hits:                              26|26                   (No change)
  Total read+write:                  118852|125668               (-5.42382%) [-1.05735x]
  Estimated Cycles:                  119764|126580               (-5.38474%) [-1.05691x]
icount::icount_bench_fdimf16_group::icount_bench_fdimf16 logspace:setup_fdimf16()
  Baselines:                      softfloat|softfloat (old)
  Instructions:                       43576|48608                (-10.3522%) [-1.11548x]
  L1 Hits:                            54766|62976                (-13.0367%) [-1.14991x]
  LL Hits:                                3|1                    (+200.000%) [+3.00000x]
  RAM Hits:                               9|9                    (No change)
  Total read+write:                   54778|62986                (-13.0315%) [-1.14984x]
  Estimated Cycles:                   55096|63296                (-12.9550%) [-1.14883x]
icount::icount_bench_fmaf128_group::icount_bench_fmaf128 logspace:setup_fmaf128()
  Baselines:                      softfloat|softfloat (old)
  Instructions:                      169465|173528               (-2.34141%) [-1.02398x]
  L1 Hits:                           220412|227678               (-3.19135%) [-1.03297x]
  LL Hits:                               55|62                   (-11.2903%) [-1.12727x]
  RAM Hits:                              63|63                   (No change)
  Total read+write:                  220530|227803               (-3.19267%) [-1.03298x]
  Estimated Cycles:                  222892|230193               (-3.17169%) [-1.03276x]
icount::icount_bench_fmaxf16_group::icount_bench_fmaxf16 logspace:setup_fmaxf16()
  Baselines:                      softfloat|softfloat (old)
  Instructions:                       34123|37300                (-8.51743%) [-1.09310x]
  L1 Hits:                            44258|49557                (-10.6927%) [-1.11973x]
  LL Hits:                                4|0                    (+++inf+++) [+++inf+++]
  RAM Hits:                               5|5                    (No change)
  Total read+write:                   44267|49562                (-10.6836%) [-1.11962x]
  Estimated Cycles:                   44453|49732                (-10.6149%) [-1.11875x]
icount::icount_bench_rintf16_group::icount_bench_rintf16 logspace:setup_rintf16()
  Baselines:                      softfloat|softfloat (old)
  Instructions:                       84874|95376                (-11.0112%) [-1.12374x]
  L1 Hits:                           100480|117431               (-14.4349%) [-1.16870x]
  LL Hits:                                1|1                    (No change)
  RAM Hits:                              10|11                   (-9.09091%) [-1.10000x]
  Total read+write:                  100491|117443               (-14.4342%) [-1.16869x]
  Estimated Cycles:                  100835|117821               (-14.4168%) [-1.16845x]

There were some notable regressions:

icount::icount_bench_u128_group::icount_bench_u256_shl linspace:setup_u256_shift()
Performance has regressed: Instructions (3110513 -> 3878511) regressed by +24.6904% (>+5.00000%)
  Baselines:                      softfloat|softfloat (old)
  Instructions:                     3878511|3110513              (+24.6904%) [+1.24690x]
  L1 Hits:                          5625738|4128141              (+36.2778%) [+1.36278x]
  LL Hits:                            57612|57613                (-0.00174%) [-1.00002x]
  RAM Hits:                              12|12                   (No change)
  Total read+write:                 5683362|4185766              (+35.7783%) [+1.35778x]
  Estimated Cycles:                 5914218|4416626              (+33.9081%) [+1.33908x]
icount::icount_bench_u128_group::icount_bench_u256_shr linspace:setup_u256_shift()
Performance has regressed: Instructions (3110513 -> 3878511) regressed by +24.6904% (>+5.00000%)
  Baselines:                      softfloat|softfloat (old)
  Instructions:                     3878511|3110513              (+24.6904%) [+1.24690x]
  L1 Hits:                          5625738|4128141              (+36.2778%) [+1.36278x]
  LL Hits:                            57612|57613                (-0.00174%) [-1.00002x]
  RAM Hits:                              12|12                   (No change)
  Total read+write:                 5683362|4185766              (+35.7783%) [+1.35778x]
  Estimated Cycles:                 5914218|4416626              (+33.9081%) [+1.33908x]