stand: Add a snarky note about the upstream ZFS situation

The latest import of openzfs broke the hacks that we used to omit the
special registers being used on arm64. Add snarky note documenting this
situation since it's a mess now since the hack was only partially
undone, leaving behind a mess.

Sponsored by:		Netflix

stand/libsa/zfs/Makefile.inc

@@ -19,6 +19,7 @@ ZSTD_SRC+=	zstd_common.c
 ZSTD_SRC+=	zstd_ddict.c zstd_decompress.c zstd_decompress_block.c
 ZSTD_SRC+=	zstd_double_fast.c zstd_fast.c zstd_lazy.c zstd_ldm.c
 
+# This is completely bogus: We should be able to omit this code completely.
 .if ${MACHINE_ARCH} == "aarch64"
 ZFS_SRC_AS =	b3_aarch64_sse2.S b3_aarch64_sse41.S
 .endif
@@ -90,10 +91,13 @@ CFLAGS.skein_block.c+=	-DSKEIN_LOOP=111
 
 # To find blake3_impl.c in OpenZFS tree for our somehat ugly blake3_impl_hack.c
 # that's needed until the necessary tweaks can be upstreamed.
+# XXX the last import gutted all this since upstream changes broke this hack.
 CFLAGS.blake3_impl_hack.c+= -I${OZFS}/module/icp/algs/blake3 -I${OZFS}/module/icp/include
 
 CWARNFLAGS.zfs.c+= ${NO_WDANGLING_POINTER}
 
+# Needing to remove the -mgeneral-regs-only is a red flag that this is not quite
+# right. But it's needed at the moment due to the muddled upstream.
 b3_aarch64_sse2.o: b3_aarch64_sse2.S
 	${CC} -c ${CFLAGS:N-mgeneral-regs-only} ${WERROR} ${.IMPSRC} \
 	    -o ${.TARGET}