Rationalize specification of cross-build target tools

[vadimcn]

Currently android cross-builds use a special switch, --android-cross-path, to specify the location of android SDK, whereas all other cross-builds seem to rely on overriding the linker using the --linker= switch. There should be only one way...

Also, currently we don't seem to be using target toolchain's ar, except for android.

[alexcrichton]

I was thinking about this recently as well. We do allow overriding ar via the --ar flag, but I think we should consolidate these:

• --linker - linker to run, highest priority
• --ar - archive tool to run, highest priority
• --cross-path - root to the directory with the cross-compiler toolchain, less priority than the previous options
• no options - lowest priority, just run gcc and ar.

I've found that most cross-compile toolchains are all rooted in a directory somewhere, so I think this would suit situations other than android nicely (while still allowing specific overrides).

[errordeveloper]

I think this needs closing in the light of rust-lang/rfcs#131.

[steveklabnik]

Agreed.