Revert "MIPS: support mips*64 as CPU and gnuabi64 as ABI"

This reverts commit 32f1c80375ebe8ad25d9805ee5889f0006c51e59.  It had
two unrelated changes lumped together, one of which changed the meaning
of the `mipsisa64*-*-linux*' target triplets, which was not properly
evaluated.