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.
    
    Change-Id: I087b243784edf6705fdaf9c32e3233da5e387283

From-SVN: r249485