unix: add NOSTD syscall numbers on FreeBSD

The syscalls marked as NOSTD are defined in kernel modules that
might be missing on some systems, but are usually present.

Fixes golang/go#27072

Change-Id: Icf9188df85b54872eb80ba73828f35aa578d49aa
Reviewed-on: https://go-review.googlesource.com/129855
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/mksysnum_freebsd.pl

@@ -27,7 +27,7 @@ const (
 EOF
 
 while(<>){
-	if(/^([0-9]+)\s+\S+\s+STD\s+({ \S+\s+(\w+).*)$/){
+	if(/^([0-9]+)\s+\S+\s+(?:NO)?STD\s+({ \S+\s+(\w+).*)$/){
 		my $num = $1;
 		my $proto = $2;
 		my $name = "SYS_$3";