libgo: remove old MIPS architecture names

libgo: remove old MIPS architecture names

    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