bsd-user: regenerate FreeBSD's system call numbers

Recreate the FreeBSD system call numbers from current sys/syscall.h. Since this
isn't guaranteed to be on all systems, continue the practice of generating it
with some variation on:
	sed -e s/SYS_/TARGET_NR_/ < $FREEBSD_SRC/sys/syscall.h > syscall_nr.h
until a more comprehensive system can be put in place.

Signed-off-by: Warner Losh <imp@FreeBSD.org>
Message-Id: <20201218205451.10559-2-imp@freebsd.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>