[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214493 91177308-0d34-0410-b5e6-96231b3b80d8