arm: remove support for iWMMX/iWMMX2 intrinsics

arm: remove support for iWMMX/iWMMX2 intrinsics

The mmintrin.h header was adjusted for GCC-14 to generate a
(suppressible) warning if it was used, saying that support would be
removed in GCC-15.

Make that come true by removing the contents of this header and
emitting an error.

At this point in time I've not removed the internal support for the
intrinsics, just the wrappers that enable access to them.  That can be
done at leisure from now on.

gcc/ChangeLog:

	* config/arm/mmintrin.h: Raise an error if this header is used.
	Remove other content.