Add SHA2 SIMD feature tests for libspl

Add SHA2 SIMD feature tests for libspl

These are added via HWCAP interface:
- zfs_neon_available() for arm and aarch64
- zfs_sha256_available() for arm and aarch64
- zfs_sha512_available() for aarch64

This one via cpuid() call:
- zfs_shani_available() for x86_64

Tested-by: Rich Ercolani <rincebrain@gmail.com>
Tested-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #13741