baaab03 add aarch64 cross testing (needed for ARM SHA acceleration) (jrakibi)
2299350 hashes: add SHA256 ARM hardware acceleration (jrakibi)

Pull request description:

  #1962 adds SIMD SHA256 intrinsics for x86 machines. However, for ARM machines we’re still falling back to `software_process_block()`, which is ~4x slower according to benchmarks I ran on my system.
  
  The code is inspired by https://github.com/noloader/SHA-Intrinsics/tree/4e754bec921a9f281b69bd681ca0065763aa911c. Variable names are intentionally kept the same for easier review and comparison, although I fixed some incorrect variable names in the original implementation (more details in noloader/SHA-Intrinsics#16).
  
  these are some benchmarks I ran on an AWS EC2 instance (t4g.small) with a Neoverse-N1 CPU: 
  
  without ARM acceleration
  
  ```
  sha256/engine_input/10
    time:   [49.947 ns 49.955 ns 49.965 ns]
    thrpt:  [190.87 MiB/s 190.91 MiB/s 190.94 MiB/s]
  
  sha256/engine_input/1024
    time:   [4.1740 µs 4.1744 µs 4.1747 µs]
    thrpt:  [233.92 MiB/s 233.94 MiB/s 233.96 MiB/s]
  
  sha256/engine_input/65536
    time:   [266.68 µs 266.71 µs 266.75 µs]
    thrpt:  [234.31 MiB/s 234.34 MiB/s 234.36 MiB/s]
  ```
  
  with ARM
  ```
  sha256/engine_input/10
    time:   [16.928 ns 16.930 ns 16.931 ns]
    thrpt:  [563.26 MiB/s 563.31 MiB/s 563.36 MiB/s]
  
  sha256/engine_input/1024
    time:   [875.00 ns 875.07 ns 875.14 ns]
    thrpt:  [1.0897 GiB/s 1.0898 GiB/s 1.0899 GiB/s]
  
  sha256/engine_input/65536
    time:   [55.939 µs 55.956 µs 55.979 µs]
    thrpt:  [1.0903 GiB/s 1.0908 GiB/s 1.0911 GiB/s]
  ```
  that’s almost ~5x faster for larger blocks


ACKs for top commit:
  apoelstra:
    ACK baaab03; successfully ran local tests; though I do not have an aarch64 machine. I reviewed the code to the extent of checking that it looks like a hash function implementation
  tcharding:
    code review ACK baaab03 - looks ok when compared to other code in the file. The tests passing speaks for the correctness AFAIU. No further understanding implied and no local testing done by me.


Tree-SHA512: ec5e54dfa92991727ebae80b42e4e9e96be55db17c1288587e548352c3b4e01016f2102accf5b766bcf5b088d4d85621d9d53f19d678b9c477c4ac72e9bc8249