cmd/link: batch writing of bytes

In best of 10, linking cmd/go shows a ~10% improvement.

tip:              real  0m1.152s user  0m1.005s
this:             real  0m1.065s user  0m0.924s

Change-Id: I303a20b94332feaedc1033c453247a0e4c05c843
Reviewed-on: https://go-review.googlesource.com/19978
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/link/internal/ld/lib.go

@@ -1850,6 +1850,10 @@ func Cseek(p int64) {
 	coutbuf.off = p
 }
 
+func Cwritestring(s string) {
+	coutbuf.WriteString(s)
+}
+
 func Cwrite(p []byte) {
 	coutbuf.Write(p)
 }