syscall: add explicit build tags

Auto-generated using the following bash script:
for i in z*_*_*.go; do
        goosgoarch=`basename ${i/${i/_*/}_/} .go`
        goos=${goosgoarch/_*/}
        goarch=${goosgoarch/*_/}
        echo $i $goos $goarch
        [ "$goos" = "windows" ] && continue
        sed -i -e "/^package /i\/\/ +build $goarch,$goos\n" "$i"
done

Change-Id: I756fee551d1698080e4591fed8f058ae0450aaa5
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/10113
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/src/syscall/zsysnum_darwin_arm.go b/src/syscall/zsysnum_darwin_arm.go
index 1a53f13..1d76861 100644
--- a/src/syscall/zsysnum_darwin_arm.go
+++ b/src/syscall/zsysnum_darwin_arm.go
@@ -1,6 +1,8 @@
 // mksysnum_darwin.pl /usr/include/sys/syscall.h
 // MACHINE GENERATED BY THE ABOVE COMMAND; DO NOT EDIT
 
+// +build arm,darwin
+
 package syscall
 
 const (