Aarch64 scrypt ASM

[Diatrus]

It would be beneficial with the introduction of arm64 Macs to add native scrypt code for aarch64.

[ryandesign]

Yes because building cpuminer 2.5.1 on Apple Silicon currently fails with:

Undefined symbols for architecture arm64:
  "_sha256_init_4way", referenced from:
      _scanhash_scrypt in minerd-scrypt.o
  "_sha256_transform_4way", referenced from:
      _scanhash_scrypt in minerd-scrypt.o
  "_sha256_use_4way", referenced from:
      _scanhash_sha256d in minerd-sha2.o
      _scanhash_scrypt in minerd-scrypt.o
  "_sha256d_ms_4way", referenced from:
      _scanhash_sha256d in minerd-sha2.o
ld: symbol(s) not found for architecture arm64

[risner]

https://simd-everywhere.github.io/blog/2020/06/22/transitioning-to-arm-with-simde.html

This should make it rather easy to support Intel and Apple M1?