[mips] Correct alignment of vectors passed in varargs for the O32 ABI.

Summary:
Vectors are normally 16-byte aligned, however the O32 ABI enforces a
maximum alignment of 8-bytes since the base of the stack is 8-byte aligned.
Previously, this was enforced on the caller side, but not on the callee side.

This fixes the output of OpenCL's printf when given vectors.

Reviewers: atanasyan

Reviewed By: atanasyan

Subscribers: llvm-commits, pekka.jaaskelainen

Differential Revision: http://reviews.llvm.org/D5433