cmd/compile: omit X:framepointer in compile version

Framepointer is the default now. Only print an X: list
if the settings are _not_ the default.

Before:

$ go tool compile -V
compile version devel +a5f30d9508 Sun Jul 16 14:43:48 2017 -0400 X:framepointer
$ go1.8 tool compile -V
compile version go1.8 X:framepointer
$

After:

$ go tool compile -V
compile version devel +a5f30d9508 Sun Jul 16 14:43:48 2017 -0400
$ go1.9 tool compile -V # imagined
compile version go1.9
$

Perpetuates #18317.

Change-Id: I981ba5c62be32e650a166fc9740703122595639b
Reviewed-on: https://go-review.googlesource.com/49252
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -104,7 +104,7 @@ func hidePanic() {
 
 func doversion() {
 	p := objabi.Expstring()
-	if p == "X:none" {
+	if p == objabi.DefaultExpstring() {
 		p = ""
 	}
 	sep := ""

src/cmd/internal/objabi/util.go

@@ -46,7 +46,6 @@ func Getgoextlinkenabled() string {
 }
 
 func init() {
-	framepointer_enabled = 1 // default
 	for _, f := range strings.Split(goexperiment, ",") {
 		if f != "" {
 			addexp(f)
@@ -80,7 +79,7 @@ func addexp(s string) {
 }
 
 var (
-	framepointer_enabled     int
+	framepointer_enabled     int = 1
 	Fieldtrack_enabled       int
 	Preemptibleloops_enabled int
 	Clobberdead_enabled      int
@@ -100,6 +99,12 @@ var exper = []struct {
 	{"clobberdead", &Clobberdead_enabled},
 }
 
+var defaultExpstring = Expstring()
+
+func DefaultExpstring() string {
+	return defaultExpstring
+}
+
 func Expstring() string {
 	buf := "X"
 	for i := range exper {

test/nosplit.go

@@ -198,6 +198,10 @@ func main() {
 		goarch = runtime.GOARCH
 	}
 
+	// Frame pointer is on by default now.
+	// golang.org/issue/18317.
+	return
+
 	version, err := exec.Command("go", "tool", "compile", "-V").Output()
 	if err != nil {
 		bug()