jpeg-turbo: add simd support for aarch64

jpeg-turbo: add simd support for aarch64

>From the 1.5-b1 release notes:

Added ARM 64-bit (ARMv8) NEON SIMD implementations of the commonly-used
compression algorithms (including the slow integer forward DCT and h2v2 &
h2v1 downsampling algorithms, which are not accelerated in the 32-bit NEON
implementation.) This speeds up the compression of full-color JPEGs by about
75% on average on a Cavium ThunderX processor and by about 2-2.5x on average
on Cortex-A53 and Cortex-A57 cores.

Add it unconditionally for all aarch64 cores, as neon support is required
for all "standard" ARMv8 implementations.  If an ARMv8 implementation w/o
NEON ever shows up, then we will need to add a BR2_AARCH64_CPU_HAS_NEON and
handle it like ARM.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>