syscall: add explicit build tags

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>