[mips] Treat -mcpu=generic the same way as an empty CPU string.

Summary:
This should fix the MCJIT unit tests that were broken by r201792 on the MIPS buildbot.
MIPS currently uses the default implementation of sys::getHostCPUName() which
always returns "generic". For now, we will accept "generic" and coerce it to
"mips32" or "mips64" depending on the target architecture like we do for empty
CPU names.

Reviewers: jacksprat, matheusalmeida

Reviewed By: jacksprat

Differential Revision: http://llvm-reviews.chandlerc.com/D2878

llvm-svn: 202253