Add mxu flag for sub-projects that use MXU inline assembly.

This avoids the need to use this flag globally in
build/core/combo/arch/mips/mips32r2-fp-xburst.mk

Change-Id: I847de04140b01f9cd0f83ff9882651fd344cd0f4
Signed-off-by: Paul Lind <paul.lind@imgtec.com>