Turn off zstd on aarch64

loader support for zstd and zfs doesn't work for aarch64. Disable it
to unbreak the build.

stand/libsa/zfs/Makefile.inc

@@ -4,7 +4,11 @@
 .PATH:		${SYSDIR}/crypto/skein
 .PATH:		${ZFSOSSRC}/spl
 .PATH:		${OZFS}/module/zstd/lib
-ZFSSRC=		zfs.c nvlist.c skein.c skein_block.c list.c zstd_shim.c zstd.c
+ZFSSRC=		zfs.c nvlist.c skein.c skein_block.c list.c
+.if ${MACHINE_ARCH} != aarch64
+ZFSSRC+=	zstd_shim.c zstd.c
+CFLAGS+=	-DHAS_ZSTD_ZFS
+.endif
 SRCS+=		${ZFSSRC}
 
 CFLAGS+=	-I${LDRSRC}

stand/libsa/zfs/zfsimpl.c

@@ -42,7 +42,9 @@ __FBSDID("$FreeBSD$");
 #include "zfsimpl.h"
 #include "zfssubr.c"
 
+#ifdef HAS_ZSTD_ZFS
 extern int zstd_init(void);
+#endif
 
 struct zfsmount {
 	const spa_t	*spa;
@@ -130,7 +132,9 @@ static const char *features_for_read[] = {
 	"com.delphix:device_removal",
 	"com.delphix:obsolete_counts",
 	"com.intel:allocation_classes",
+#ifdef HAS_ZSTD_ZFS
 	"org.freebsd:zstd_compress",
+#endif
 	NULL
 };
 
@@ -171,7 +175,9 @@ zfs_init(void)
 	dnode_cache_buf = malloc(SPA_MAXBLOCKSIZE);
 
 	zfs_init_crc();
+#ifdef HAS_ZSTD_ZFS
 	zstd_init();
+#endif
 }
 
 static int

sys/cddl/boot/zfs/zfssubr.c

@@ -105,9 +105,10 @@ typedef struct zio_checksum_info {
 #include "sha256.c"
 #include "skein_zfs.c"
 
+#ifdef HAS_ZSTD_ZFS
 extern int zfs_zstd_decompress(void *s_start, void *d_start, size_t s_len,
     size_t d_len, int n);
-
+#endif
 
 static zio_checksum_info_t zio_checksum_table[ZIO_CHECKSUM_FUNCTIONS] = {
 	{{NULL, NULL}, NULL, NULL, 0, "inherit"},
@@ -183,7 +184,9 @@ static zio_compress_info_t zio_compress_table[ZIO_COMPRESS_FUNCTIONS] = {
 	{NULL,			NULL,			9,	"gzip-9"},
 	{NULL,			zle_decompress,		64,	"zle"},
 	{NULL,			lz4_decompress,		0,	"lz4"},
+#ifdef HAS_ZSTD_ZFS
 	{NULL,			zfs_zstd_decompress, ZIO_ZSTD_LEVEL_DEFAULT, "zstd"}
+#endif
 };
 
 static void