[mips] Fix va_arg() for pointer types on big-endian N32.

Summary:
The Mips ABI's treat pointers in the same way as integers. They are
sign-extended to 32-bit for O32, and 64-bit for N32/N64. This doesn't matter
for O32 and N64 where pointers are already the correct width but it does matter
for big-endian N32, where pointers are 32-bit and need promoting.

The caller side is already passing pointers correctly. This patch corrects the
callee.

Reviewers: vmedic, atanasyan

Reviewed By: atanasyan

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6812

llvm-svn: 225782