arm: warn about deprecation of iwmmx in mmintrin.h

GCC 13's changes file documents that iwmmx is deprecated.  Raise the bar
by warning when the mmintrin.h header is included by users, but provide
a way to suppress the warning.

gcc:
	* config/arm/mmintrin.h: Warn if this header is included without
	defining __ENABLE_DEPRECATED_IWMMXT.