Add AArch64 ARM Neon code (complements #1823)

[mastertheknife]

Pull request #1823 added AArch32 ARM Neon versions of the Blend() and Delta() functions, which are used by zoneminder's motion detection.
This pull requests complements pull request #1823 by adding AArch64 ARM Neon versions of the same functions, so both AArch32 and AArch64 are now available.
This pull request also includes a minor change to the AArch32 neon functions: relocate the data prefetches to be after the loads, to better utilize memory bus bandwidth.

In AArch64 mode, Neon is a mandatory feature of ARMv8-A CPUs, so no compiler flags are needed and no runtime detection is is needed. Neon is assumed to be always available.
Performance is pretty much identical to the AArch32 versions, but a comparison is provided:

Odroid C2 with ARM Cortex A53 processor @ 1.5 GHz:

Function	Std 32bit (-O2)	Std 64bit (-O2)	Neon (AArch32)	Neon (AArch64)
8bit Delta	215 MPixels/sec	258 MPixels/sec	968 MPixels/sec	968 MPixels/sec
32bit Delta	67 MPixels/sec	79 MPixels/sec	309 MPixels/sec	309 MPixels/sec
Fastblend	188 MColors/sec	188 MColors/sec	1130 MColors/sec	1130 MColors/sec

Scaleway ARM64-2GB instance with 2 cores of Cavium ThunderX processor:

Function	Std 64bit (-O2)	Neon (AArch64)
8bit Delta	174 MPixels/sec	2176 MPixels/sec
32bit Delta	68 MPixels/sec	373 MPixels/sec
Fastblend	177 MColors/sec	1757 MColors/sec

It seems the ThunderX loves Neon, or isn't as memory bound as the Odroid C2 is.

The CPU reduction should be between 20-50%. Perhaps in the future i will work on creating ARM Neon version of AlarmedPixels, which is currently the biggest CPU consumer in zma.