ARM64 NEON SIMD implementation of Huffman encoding

ARM64 NEON SIMD implementation of Huffman encoding

Full-color compression speedups relative to previous commits:
Cortex-A53 (Nexus 5X), Android, 64-bit: 1.1-13% (avg. 6.0%)
Cortex-A57 (Nexus 5X), Android, 64-bit: 0.0-22% (avg. 6.3%)

Refer to #47 and #50 for discussion

Closes #50

Note that this commit introduces a similar /proc/cpuinfo parser to that
of the ARM32 implementation.  It is used to specifically check whether
the code is running on Cavium ThunderX and, if so, disable the ARM64
SIMD Huffman routines (which slow performance by an average of 8% on
that CPU.)

Based on:
https://github.com/mayeut/libjpeg-turbo/commit/a8c282e5e5ac10a715d6d6a9ab22121982b485f6