[mips] Add assembler support for .set msa/nomsa directive.

Summary:
These directives are used to toggle whether the assembler accepts MSA-specific instructions or not.

Patch by Matheus Almeida and Toma Tabacu.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D4783

llvm-svn: 215099