cmd/goyacc: put //line annotations on the first column.

This appears to have broken since go1.11, and "go doc compile" and
https://golang.org/issue/24183 documents this new convention (point
2)

Change-Id: I6a4bc0fc51a18c17ac9a72a67d9a8eb4c879e0f9
Reviewed-on: https://go-review.googlesource.com/137515
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

cmd/goyacc/yacc.go

@@ -1272,7 +1272,7 @@ l1:
 func cpyact(curprod []int, max int) {
 
 	if !lflag {
-		fmt.Fprintf(fcode, "\n\t\t//line %v:%v", infile, lineno)
+		fmt.Fprintf(fcode, "\n//line %v:%v", infile, lineno)
 	}
 	fmt.Fprint(fcode, "\n\t\t")