[Mips] MSA frontend option support

This patch adds -mmsa and -mno-msa to the options supported by 
clang to enable and disable support for MSA.

When MSA is enabled, a predefined macro '__mips_msa' is defined to 1.

Patch by Daniel Sanders


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188184 91177308-0d34-0410-b5e6-96231b3b80d8