[libc] Build two different static archives libc.a and libm.a under full build.

We currently put everything in one single archive libc.a which breaks in
certain situations where the compiler drivers expect libm.a also. With
this change, we separate out libc.a and libm.a functions as is done
conventionally and put them in two different static archives.

One will now have to build two targets, `libc` and `libm` which produce
`libc.a` and `libm.a` respectively. Under default build, one still builds only
one target named `libc` which produces `libllvmlibc.a`.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D143005