[mips] Add macros _MIPS_ISA and __mips_isa_rev (same expansion as defined by GCC).

Summary: The Linux Kernel is one example of a piece of software that relies on them.

Reviewers: atanasyan

Reviewed By: atanasyan

Differential Revision: http://reviews.llvm.org/D3756

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210270 91177308-0d34-0410-b5e6-96231b3b80d8