Always over-specify mips. Makes life easier when the cross compiler
isn't configured quite correctly and outputs the wrong thing by
default. Introduce _ABI_CFLAGS to hold the over specification. Use
_ABI_CFLAGS in COPTFLAGS in the kernel build, and add those flags
to the various FOO_CFLAGS definitions.
XXX haven't made a pass through files to pick up the crazy, loony
XXX stuff that goes on there.