[mips] Support building compiler-rt with gcc.
This will allow us to enable it on the buildbot (and then enable a
The problem is that gcc for Mips does not accept -m32/-m64 like clang does.
We therefore need to use the nearest equivalent -mips32r2/-mips64r2. We must
also specify -mabi=64 in the -mips64r2 case since gcc's default ABI has
traditionally been N32 whereas clang's is N64.
Reviewed By: sagar
Subscribers: llvm-commits, samsonov, kcc, mohit.bhakkad, Anand.Takale, sagar
Differential Revision: http://reviews.llvm.org/D8892