Clang-side extension of D26344
Exposes PPC target to various small data threshold arguments. Also extends alternate forms to other MIPS uses.
As of r293254, the -G= and -msmall-data-threshold= flags are just aliases of -G, so you don't need those parts of this patch any more. The PPC part looks fine, but please add a testcase.
In future, please add cfe-commits as a subscriber to Clang changes; otherwise mail doesn't get sent there and patches tend to get lost.