[IAS] Teach -cc1as about the 'target-abi' option.

Summary:
When using the IAS from clang, the 'target-abi' option gets passed to cc1as, but cc1as doesn't know about it and gives an "unknown argument" error.

This is fixed by adding the 'CC1AsOption' flag to the 'target-abi' option in CC1Options.td.

Reviewers: atanasyan, echristo, dsanders

Reviewed By: dsanders

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7903

llvm-svn: 231244