[mips] Separated mips specific -Wa options, so that they are not checked on other platforms.

Summary: This is a follow on to post review comments on revision r248276.

Patch by Scott Egerton.

Reviewers: vkalintiris, dsanders

Subscribers: joerg, rengolin, cfe-commits

Differential Revision: http://reviews.llvm.org/D13100

llvm-svn: 251430