cmd/compile: mark a few more tests as parallel

Reduces the time on my machine for

go clean -cache; go test -short -count=1 cmd/compile/internal/gc

from 4.7s to 3.7s.

Updates #26473

Change-Id: I9f9573675ffd6519da63961f48f61260ae4717fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/176937
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -14,6 +14,7 @@ import (
 
 func TestBuiltin(t *testing.T) {
 	testenv.MustHaveGoRun(t)
+	t.Parallel()
 
 	old, err := ioutil.ReadFile("builtin.go")
 	if err != nil {

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

@@ -19,6 +19,7 @@ import (
 // fmt.scanf routines. See issue 6853.
 func TestScanfRemoval(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
+	t.Parallel()
 
 	// Make a directory to work in.
 	dir, err := ioutil.TempDir("", "issue6853a-")
@@ -66,6 +67,7 @@ func main() {
 // Make sure -S prints assembly code. See issue 14515.
 func TestDashS(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
+	t.Parallel()
 
 	// Make a directory to work in.
 	dir, err := ioutil.TempDir("", "issue14515-")

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

@@ -207,6 +207,7 @@ const detailOutput = false
 // corresponds to what we expect it to be.
 func TestScopeRanges(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
+	t.Parallel()
 
 	if runtime.GOOS == "plan9" {
 		t.Skip("skipping on plan9; no DWARF symbol table in executables")
@@ -445,6 +446,7 @@ func gobuild(t *testing.T, dir string, optimized bool, testfile []testline) (str
 // See issue #23928.
 func TestEmptyDwarfRanges(t *testing.T) {
 	testenv.MustHaveGoRun(t)
+	t.Parallel()
 
 	if runtime.GOOS == "plan9" {
 		t.Skip("skipping on plan9; no DWARF symbol table in executables")