[Mips] Handle -mips32r[3|5] / -mips64r[3|5] options while selecting lib/headers paths

There is no supported toolchain which provides headers / libs / object
files specific to the mips32r[3|5] and mips64r[3|5] ISA. So select "r2"
specific folders when they are available.

http://reviews.llvm.org/D7879

llvm-svn: 230611