[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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204917 91177308-0d34-0410-b5e6-96231b3b80d8