neon vectorization for base64

A similar algorithm is used to vectorize on x86_64, with a good description in
https://arxiv.org/abs/1704.00605 . On AArch64 the implementation differs in that
instead of using multiplies to shift bits around, it uses the vld3+vst4 and
vld4+vst3 combinations to load and store interleaved data.  This patch is based
on the NEON implementation of Wojciech Mula:
https://github.com/WojciechMula/base64simd/blob/master/encode/encode.neon.cpp
https://github.com/WojciechMula/base64simd/blob/master/encode/lookup.neon.cpp
and
https://github.com/WojciechMula/base64simd/blob/master/encode/encode.neon.cpp
https://github.com/WojciechMula/base64simd/blob/master/encode/encode.neon.cpp
adapted to php/ext/standard/base64.c and vectorized with factor 16 instead of 8.

On a Graviton A1 instance and on the synthetic benchmarks in
https://github.com/lemire/fastbase64 I see 175% speedup on base64 encoding and
60% speedup on base64 decode compared to the scalar implementation.

The patch passes `make test` regression testing on aarch64-linux.

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