[mips] implement .set dspr2 directive

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314994 91177308-0d34-0410-b5e6-96231b3b80d8