Mips target in gold

[Cary Coutant]

> Please find attached first of the patches that implement Mips target in gold. This patch contains first part of the Mips specific changes (mips.cc file is split in two patches). Mips32, mips16 and micromips architectures are currently supported.

Thanks for working on this. Because of the size of the patches, it'll
take me a while to review. I'll try to look at it over the next couple
of weeks.

-cary