cmd/internal/gc, cmd/internal/obj: remove pointless fmt.Sprintf calls

This is a follow-up to CL 6265. No behavior changes.
The diff was generated with eg, using template:

package p

import "fmt"

func before(a string) string { return fmt.Sprintf(a) }
func after(a string) string  { return a }

Change-Id: I7b3bebf31be5cd1ae2233da06cb4502a3d73f092
Reviewed-on: https://go-review.googlesource.com/6269
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/cmd/internal/gc/walk.go b/src/cmd/internal/gc/walk.go
index b242fd4..c57bdd6 100644
--- a/src/cmd/internal/gc/walk.go
+++ b/src/cmd/internal/gc/walk.go
@@ -1796,13 +1796,13 @@
 	var savel Iter
 
 	fmt_ := ""
-	fmt_ += fmt.Sprintf("\t")
+	fmt_ += "\t"
 	first := 1
 	for l := Structfirst(&savel, nl); l != nil; l = structnext(&savel) {
 		if first != 0 {
 			first = 0
 		} else {
-			fmt_ += fmt.Sprintf(", ")
+			fmt_ += ", "
 		}
 		fmt_ += fmt.Sprintf("%v", Tconv(l, 0))
 	}
@@ -1817,14 +1817,14 @@
 	var r *Node
 
 	fmt_ := ""
-	fmt_ += fmt.Sprintf("\t")
+	fmt_ += "\t"
 	first := 1
 	for ; l != nil; l = l.Next {
 		r = l.N
 		if first != 0 {
 			first = 0
 		} else {
-			fmt_ += fmt.Sprintf(", ")
+			fmt_ += ", "
 		}
 		fmt_ += fmt.Sprintf("%v", Tconv(r.Type, 0))
 	}