Miscellaneous FreBSD compilation bugfixes

Add missing machine/md_var.h to spl/sys/simd_aarch64.h and
spl/sys/simd_arm.h

In spl/sys/simd_x86.h, PCB_FPUNOSAVE exists only on amd64, use PCB_NPXNOSAVE
on i386

In FreeBSD sys/elf_common.h redefines AT_UID and AT_GID on FreeBSD, we need
a hack in vnode.h similar to Linux. sys/simd.h needs to be included early.

In zfs_freebsd_copy_file_range() we pass a (size_t *)lenp to
zfs_clone_range() that expects a (uint64_t *)

Allow compiling armv6 world by limiting ARM macros in sha256_impl.c and
sha512_impl.c to __ARM_ARCH > 6

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Reviewed-by: Signed-off-by: WHR <msl0000023508@gmail.com>
Signed-off-by: Martin Matuska <mm@FreeBSD.org>
Closes #14674