[mips][mips64r6] Implement -mips32r6 and -mips64r6 aliases to -march=mips32r6 and -march=mips64r6

Differential Revision: http://reviews.llvm.org/D4434


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212625 91177308-0d34-0410-b5e6-96231b3b80d8