[mips] implement .set dspr2 directive

Implement .set dspr2 directive with appropriate feature bits. This
directive is a counterpart of -mattr=dspr2 command line option with the
exception that it does not influence elf header flags.

Patch by Milos Stojanovic.

Differential Revision: https://reviews.llvm.org/D38537

llvm-svn: 314994