[mips] Support micromips attribute passed by front-end

This patch adds handling of the `micromips` and `nomicromips` attributes
passed by front-end. The patch depends on D33363.

Differential revision: https://reviews.llvm.org/D33364

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303545 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed