[mips] Always clobber $1 for MIPS inline asm.

Summary:
Because GCC doesn't use $1 for code generation, inline assembly code can use $1 without having to add it to the clobbers list.

LLVM, on the other hand, does not shy away from using $1, and this can cause conflicts with inline assembly which assumes GCC-like code generation.

A solution to this problem is to make Clang automatically clobber $1 for all MIPS inline assembly.
This is not the optimal solution, but it seems like a necessary compromise, for now.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6638