Files
FFmpeg/libavutil
Shreesh Adiga 58a2dc3c2e avutil/base64: add aarch64 NEON implementation for base64 encode
Add NEON implementation using LD3 to load 48 bytes with de-interleaving
using which 6 bits are extracted using USHR and SLI to get 4 registers, which
is used as a index into lookup table using TBL with 4 registers to translate
into the 64-byte output, which is written using ST4 to interleave.
Add a checkasm test to validate the input lengths from 0 to 239 bytes to cover
all the code paths introduced in the NEON implementation.
Refactored the C implementation into a header file so that it can be used
from checkasm test without having to duplicate the code there.

Observed about 4x speedup by checkasm on A720 core on 1kB input:
C:
 - base64.base64 [OK]
NEON:
 - base64.base64 [OK]
checkasm: all 1 tests passed
Benchmark results:
  name                      nsecs (vs ref)
  av_base64_encode_c:      1286.9
  av_base64_encode_neon:    315.5 ( 4.08x)
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