unix: add //go:build lines when generating files

The //+go:build lines were added by running Go 1.17 gofmt in CL 294490.
Make sure to retain them when re-generating files in this package.

For golang/go#41184.

Change-Id: Ia418b8c0eb4291a0db3f4bb97685bde78d0ff016
Reviewed-on: https://go-review.googlesource.com/c/sys/+/296889
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/mkmerge.go

@@ -466,6 +466,7 @@ func merge(mergedFile string, archFiles ...string) error {
 	buf := bufio.NewWriter(f)
 	fmt.Fprintln(buf, "// Code generated by mkmerge.go; DO NOT EDIT.")
 	fmt.Fprintln(buf)
+	fmt.Fprintf(buf, "//go:build %s\n", goos)
 	fmt.Fprintf(buf, "// +build %s\n", goos)
 	fmt.Fprintln(buf)
 	buf.Write(mergedSrc)