[MIPS] Use ABI to determine stack alignment.

Summary:
The stack alignment depends on the ABI (16 bytes for N32 and N64 and 8
bytes for O32), not the CPU type.

Reviewers: sdardis

Reviewed By: sdardis

Subscribers: atanasyan, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D36326

llvm-svn: 310768