Add generic implementation handling and SHA2 impl

The skeleton file module/icp/include/generic_impl.c can be used for
iterating over different implementations of algorithms.

It is used by SHA256, SHA512 and BLAKE3 currently.

The Solaris SHA2 implementation got replaced with a version which is
based on public domain code of cppcrypto v0.10.

These assembly files are taken from current openssl master:
- sha256-x86_64.S: x64, SSSE3, AVX, AVX2, SHA-NI (x86_64)
- sha512-x86_64.S: x64, AVX, AVX2 (x86_64)
- sha256-armv7.S: ARMv7, NEON, ARMv8-CE (arm)
- sha512-armv7.S: ARMv7, NEON (arm)
- sha256-armv8.S: ARMv7, NEON, ARMv8-CE (aarch64)
- sha512-armv8.S: ARMv7, ARMv8-CE (aarch64)
- sha256-ppc.S: Generic PPC64 LE/BE (ppc64)
- sha512-ppc.S: Generic PPC64 LE/BE (ppc64)
- sha256-p8.S: Power8 ISA Version 2.07 LE/BE (ppc64)
- sha512-p8.S: Power8 ISA Version 2.07 LE/BE (ppc64)

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

include/Makefile.am

@@ -75,6 +75,7 @@ COMMON_H = \
 	sys/rrwlock.h \
 	sys/sa.h \
 	sys/sa_impl.h \
+	sys/sha2.h \
 	sys/skein.h \
 	sys/spa.h \
 	sys/spa_checkpoint.h \
@@ -124,6 +125,7 @@ COMMON_H = \
 	sys/zfs_delay.h \
 	sys/zfs_file.h \
 	sys/zfs_fuid.h \
+	sys/zfs_impl.h \
 	sys/zfs_project.h \
 	sys/zfs_quota.h \
 	sys/zfs_racct.h \