[mips] Implement custom MCELFStreamer.

This allows us to insert some hooks before emitting data into an actual object file.
For example, we can capture the register usage for a translation unit by overriding
the EmitInstruction method. The register usage information is needed to generate
.reginfo and .Mips.options ELF sections.

No functional changes.

Differential Revision: http://llvm-reviews.chandlerc.com/D3129