[mips] Explicitly specify the linker emulation for MIPS on FreeBSD.

[mips] Explicitly specify the linker emulation for MIPS on FreeBSD.

FreeBSD's mips64 builds O32 binaries for /usr/lib32 by default and
thus needs to be able to link O32 binaries which requires an explicit
linker emulation.  Go ahead and list all the linker emulation variants
for MIPS so that any supported MIPS ABI binary can be linked by any
linker supporting MIPS.

Reviewed By: atanasyan

Differential Revision: https://reviews.llvm.org/D48507

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@335691 91177308-0d34-0410-b5e6-96231b3b80d8