cc-wrapper: when cross building, wedge in setting AR, AS, and LD env vars to...

cc-wrapper: when cross building, wedge in setting AR, AS, and LD env vars to their cross building versions

I think it's ok to export things which aren't wrapped. The cc-wrapper
can be thought of as responsible for all of binutils and the c
compiler, only wrapping those binaries which are necessary to
interposition---as opposed to all binaries it thinks are relevaant.

Conversely, adding the setup hook to the unwrapped compilers would be
unforunate as hooks are ugly hacks and the compilers themselves take
a long time to rebuild. Better to wholely separate "pure packages" from
hacks.