[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>

core/combo/arch/mips/mips32r2-fp-xburst.mk

@@ -10,7 +10,8 @@ arch_variant_cflags := \
     -mtune=mips32r2 \
     -mips32r2 \
     -mhard-float \
-    -mno-fused-madd
+    -mno-fused-madd \
+    -Wa,-mmxu
 
 arch_variant_ldflags := \
     -EL