Add 64-bit AArch64 asimd SIMD support

This adds the low-level SIMD implementation
for the 64-bit ARM AArch64 architecture in
single and double precision. We use the
asimd (advanced SIMD) nomenclature that is
also present in the CPU flags, but this is
the same as AArch64-neon, and present on all
AArch64 hardware. Just as for the 32-bit ARM
Neon support in the parent patch this will not
generate kernels yet, and for this reason we do
not yet enable AARCH64_ASIMD by default. Unit
and regression tests pass on AArch64 hardware
with gcc-4.9.

Change-Id: I67b2615d30e2038af480270f934a6d2788c3ed7b