Zstd: Integrate v1.5.7 into the ZFS build system

This commit builds on the previous zstd library update and adds the
necessary ZFS integration and build system changes required to make
zstd 1.5.7 compile and function correctly.

Changes:
- Add zstd_preSplit.c (new in 1.5.7) to all build systems.
- Enable x86_64 assembly in userspace (huf_decompress_amd64.S).
- Disable assembly in kernel for RETHUNK/IBT compatibility.
- Disable intrinsics in kernel for EL10 x86_64-v3 baseline.
- Disable tracing in kernel builds for AArch64 compatibility.
- Fix ZSTD_isError symbol renaming with __asm__ directive.
- Rename abs64 to ZSTD_abs64 (FreeBSD kernel conflict).
- Fix bitstream.h attributes (MEM_STATIC -> FORCE_INLINE_TEMPLATE).
- Remove xxhash.c from BSD build (now header-only).
- Update symbol names in zstd_compat_wrapper.h.
- Ignore checkstyle for zstd-in.c.

Kernel assembly disabled for security mitigation compatibility. User
space retains full performance.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Moch <mail@alexmoch.com>
Closes #18089

lib/libzstd/Makefile.am

@@ -26,10 +26,16 @@ nodist_libzstd_la_SOURCES = \
 	module/zstd/lib/compress/zstd_lazy.c \
 	module/zstd/lib/compress/zstd_ldm.c \
 	module/zstd/lib/compress/zstd_opt.c \
+	module/zstd/lib/compress/zstd_preSplit.c \
 	module/zstd/lib/decompress/huf_decompress.c \
 	module/zstd/lib/decompress/zstd_ddict.c \
 	module/zstd/lib/decompress/zstd_decompress.c \
 	module/zstd/lib/decompress/zstd_decompress_block.c \
 	module/zstd/zfs_zstd.c
 
+if TARGET_CPU_X86_64
+nodist_libzstd_la_SOURCES += \
+	module/zstd/lib/decompress/huf_decompress_amd64.S
+endif
+
 libzstd_la_CFLAGS += -include $(top_srcdir)/module/zstd/include/zstd_compat_wrapper.h -fno-tree-vectorize -Wp,-w $(AM_CFLAGS)