[mips] Defer va_arg expansion to the backend.

Summary:
This patch causes clang to emit va_arg instructions to the backend instead of
expanding them into an implementation itself. The backend already implements
va_arg since this is necessary for NaCl so this patch is removing redundant
code.

Together with the llvm patch (D4556) that accounts for the effect of endianness
on the expansion of va_arg, this fixes PR19612.

Depends on D4556

Reviewers: sstankovic, dsanders

Reviewed By: dsanders

Subscribers: rnk, cfe-commits

Differential Revision: http://reviews.llvm.org/D4742