Compute CRC32C on ARM using the Crypto Extension where available

Compute CRC32C on ARM using the Crypto Extension where available

In similar vein to commit 3c6e8c123, the ARMv8 cryptography extension
has 64x64 -> 128-bit carryless multiplication instructions suitable
for computing CRC. This was tested to be around twice as fast as
scalar CRC instructions for longer inputs.

We now do a runtime check, even for builds that target "armv8-a+crc",
but those builds can still use a direct call for constant inputs,
which we assume are short.

As for x86, the MIT-licensed implementation was generated with the
"generate" program from

https://github.com/corsix/fast-crc32/

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/CANWCAZaKhE+RD5KKouUFoxx1EbUNrNhcduM1VQ=DkSDadNEFng@mail.gmail.com