This removes the old names for the 3 main MIPS ABIs: mipso32, mipsn32
and mipsn64. It also removes the mipso64 ABI which has no equivalent
architecture name in go. This ABI has been dead for sometime and I doubt
anyone will miss it.
Reviewed-on: https://go-review.googlesource.com/46154
From-SVN: r249477
