[mips] Use more conservative default CPUs for MIPS on FreeBSD.

FreeBSD defaults to mips3 for all MIPS ABIs with GCC as that is the
minimum MIPS architecture FreeBSD supports.  Use mips3 for MIPS64 and
mips2 for MIPS32 to match.

Reviewed By: atanasyan

Differential Revision: https://reviews.llvm.org/D48499

llvm-svn: 335653