[mips] Add support for ELF64-mips and the R_MIPS_32/R_MIPS_64 relocs for it.

Summary:
This fixes several test failures when building LLVM on a MIPS host.

The failures were:
    LLVM :: DebugInfo/enum.ll
    LLVM :: DebugInfo/inlined-arguments.ll
    LLVM :: DebugInfo/member-order.ll
    LLVM :: DebugInfo/namespace.ll
    LLVM :: DebugInfo/template-recursive-void.ll
    LLVM :: DebugInfo/tu-composite.ll
    LLVM :: DebugInfo/two-cus-from-same-file.ll
    LLVM :: Linker/type-unique-simple-a.ll
    LLVM :: Linker/type-unique-simple2.ll

Reviewers: jacksprat, matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D2721