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

module/zfs/zfs_impl.c

@@ -0,0 +1,61 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or https://opensource.org/licenses/CDDL-1.0.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+/*
+ * Copyright (c) 2022 Tino Reichardt <milky-zfs@mcmilk.de>
+ */
+
+#include <sys/zio_checksum.h>
+#include <sys/zfs_context.h>
+#include <sys/zfs_impl.h>
+
+#include <sys/blake3.h>
+#include <sys/sha2.h>
+
+/*
+ * impl_ops - backend for implementations of algorithms
+ */
+const zfs_impl_t *impl_ops[] = {
+	&zfs_blake3_ops,
+	&zfs_sha256_ops,
+	&zfs_sha512_ops,
+	NULL
+};
+
+/*
+ * zfs_impl_get_ops - Get the API functions for an impl backend
+ */
+const zfs_impl_t *
+zfs_impl_get_ops(const char *algo)
+{
+	const zfs_impl_t **ops = impl_ops;
+
+	if (!algo || !*algo)
+		return (*ops);
+
+	for (; *ops; ops++) {
+		if (strcmp(algo, (*ops)->name) == 0)
+			break;
+	}
+
+	ASSERT3P(ops, !=, NULL);
+	return (*ops);
+}