ARM64 NEON: Fix another ABI conformance issue

ARM64 NEON: Fix another ABI conformance issue

Based on
https://github.com/mayeut/libjpeg-turbo/commit/98a5a9dc899aa9265858a3cbe0a96289a31a1322
with wordsmithing by DRC.

In the AArch64 ABI, as in many others, it's forbidden to read/store data
below the stack pointer.  Some SIMD functions were doing just that
(stack pointer misuse) when trying to preserve callee-saved registers,
and this resulted in those registers being restored with incorrect
contents under certain circumstances.

This patch fixes that behavior, and callee-saved registers are now
stored above the stack pointer throughout the function call.  The patch
also removes register saving in places where it is unnecessary for this
ABI, or it makes use of unused scratch regiters instead of callee-saved
registers.

Fixes #97.  Closes #101.

Refer also to https://bugzilla.redhat.com/show_bug.cgi?id=1368569