MIPS: allow o32 abi with 64bit CPU and 64 abi with 32bit triple

In general, MIPS support ELF format like
   ELF 32-bit LSB relocatable, MIPS, MIPS64 rel2 version 1 (SYSV)
and Linux's VDSO uses it.

Currently clang stop CMDs like
    clang -march=mips64r2 -mabi=32

While it is not needed now, since the the backend support the combination now.

This patch also allows something like
     clang --target=mipsel-linux-gnu -mabi=64
Since the triple can convert to right 64bit one automaticly.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D146269