[mips][PR19612] Fix va_arg for big-endian mode.

Summary:
Big-endian mode was not correctly adjusting the offset for types smaller
than an ABI slot.

Fixes PR19612

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: sstankovic, llvm-commits

Differential Revision: http://reviews.llvm.org/D4556

llvm-svn: 214493