Rename the current ARM code to ARM SIMD

Rename the current ARM code to ARM SIMD

This code is only for CPUs supporting the SIMD instructions, not for all ARM
CPUs.

I stumbled above the recent commit with the ARM SIMD code while preparing a
patch that models the patch from #13445 after the MMX and SSE2 cases:

The ARM SIMD option currently uses --disable-arm, although this code is only
for CPUs >= ARMv6. That's as if one would call the option to disable the SSE2
code --disable-x86.

This patch therefore renames the configure option and the function and file
names to arm-simd/arm_simd.