Fix cross-endian interoperability of zstd

It turns out that layouts of union bitfields are a pain, and the
current code results in an inconsistent layout between BE and LE
systems, leading to zstd-active datasets on one erroring out on
the other.

Switch everyone over to the LE layout, and add compatibility code
to read both.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12008
Closes #12022

module/zstd/zstd_sparc.c

@@ -0,0 +1,11 @@
+#ifdef __sparc__
+#include <stdint.h>
+#include <sys/byteorder.h>
+#include "include/sparc_compat.h"
+uint64_t __bswapdi2(uint64_t in) {
+	return (BSWAP_64(in));
+}
+uint32_t __bswapsi2(uint32_t in) {
+	return (BSWAP_32(in));
+}
+#endif