[mips] Move expansion of .cpsetup to target streamer.

Summary:
There are two functional changes:
1) The directive is not expanded for the ASM->ASM code path.
2) If PIC is not set, there's no expansion for the ASM->OBJ code path (same behaviour as GAS).

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D3482