Only mark functions as micromips.

The GNU as behavior is a bit different and very strange. It will mark any
label that contains an instruction. We can implement that, but using the
type looks more natural since gas will not mark a function if a .word is
used to output the instructions!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199287 91177308-0d34-0410-b5e6-96231b3b80d8