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

lib/libspl/include/Makefile.am

@@ -46,7 +46,6 @@ libspl_sys_HEADERS = \
 	%D%/sys/poll.h \
 	%D%/sys/priv.h \
 	%D%/sys/processor.h \
-	%D%/sys/sha2.h \
 	%D%/sys/simd.h \
 	%D%/sys/stack.h \
 	%D%/sys/stdtypes.h \