mmx: compile on ARM for iwmmxt optimizations

mmx: compile on ARM for iwmmxt optimizations

Check in configure for at least gcc-4.6, since gcc-4.7 (and hopefully
4.6) will be the eariest version capable of compiling the _mm_*
intrinsics on ARM/iwmmxt. Even for suitable compile versions I use
_mm_srli_si64 which is known to cause unpatched compilers to fail.

Select iwmmxt at runtime only after NEON, since we expect the NEON
optimizations to be more capable and faster than iwmmxt.

Signed-off-by: Matt Turner <mattst88@gmail.com>