This patch implements .set mips32r2 directive and sets appropriate feature bits. It also introduces helper functions that are used to set and clear feature bits as necessary. This directive is a counterpart of -mips32r2 command line options with the exception that it does not influence elf header flags. The usage example is gives in test file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202807 91177308-0d34-0410-b5e6-96231b3b80d8