do_freebsd_arch_sysarch() exists in $ARCH/target_arch_sysarch.h for x86.
Call it from do_freebsd_sysarch() and remove the mostly duplicate
version in syscall.c. Future changes will move it to os-sys.c and
support other architectures.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>