[mips] The decision to use MO_GOT_PAGE and MO_GOT_OFST depends on the ABI being N32 or N64 not the arch being MIPS64

Summary: No functional change (in supported use cases)

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D3177