[MIPS] Add support for MXU instructions for Ingenic builds.

This enables an Ingenic build to use MXU asm instructions.
MXU support was just recently added:

    ASM: https://android-review.googlesource.com/63701
    GCC: https://android-review.googlesource.com/63702
    BIN: https://android-review.googlesource.com/#/c/63704/



Change-Id: I2b60567a689efa70ec064dfbb0f241a6bc61aed1
Signed-off-by: Pete Delaney <piet.delaney@imgtec.com>