[mips] Support building compiler-rt with gcc.

Summary:
This will allow us to enable it on the buildbot (and then enable a
sanitizer buildbot).

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.

Reviewers: sagar

Reviewed By: sagar

Subscribers: llvm-commits, samsonov, kcc, mohit.bhakkad, Anand.Takale, sagar

Differential Revision: http://reviews.llvm.org/D8892

llvm-svn: 235299