[driver][mips] Adjust target triple accordingly to provided ABI name

Explicitly selected MIPS ABI using the `-mabi` option implies
corresponding target triple. For 'O32' ABI it's a 32-bit target triple
like `mips-linux-gnu`. For 'N32' and 'N64' ABIs it's a 64-bit target
triple like `mips64-linux-gnu`. This patch adjusts target triple
accordingly these rules like we do for pseudo-target flags '-m64',
'-m32' etc already.

Differential revision: https://reviews.llvm.org/D52290

llvm-svn: 343169