[mips] Make mips64 the default CPU for the mips64 architecture

Summary:
This is consistent with the integrated assembler.
All mips64 codegen tests previously passed -mcpu. Removed -mcpu from
blez_bgez.ll and const-mult.ll to cover the default case.

Ideally, the two implementations of selectMipsCPU() will be merged but it's
proven difficult to find a home for the function that doesn't cause link errors.
For now, we'll hoist the common functionality into a function and mark it with
FIXME's.

Reviewers: jacksprat, matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D2830

llvm-svn: 201782