test: restore no-gogcflags build shortcut, save time

With a clean cache on a laptop
before change
time go run run.go -- . fixedbugs
  real	2m10.195s
  user	3m16.547s
  sys	1m52.939s

Or, before, directly after make.bash (the actual use case we care about)
time go run run.go -- . fixedbugs
  real	2m8.704s
  user	3m12.327s
  sys	1m49.123s

after change
time go run run.go -- . fixedbugs
  real	1m38.915s
  user	2m38.389s
  sys	1m8.490s

Tests, fortunately, still seem to pass.

Latest version of this takes the slow route for cross-compilation, which includes wasm.

Change-Id: Iad19951612defa96c4e9830bce920c5e8733834a
Reviewed-on: https://go-review.googlesource.com/c/go/+/223083
Run-TryBot: David Chase <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Cherry Zhang <[email protected]>

test/run.go

@@ -462,6 +462,10 @@ func goGcflags() string {
 	return "-gcflags=all=" + os.Getenv("GO_GCFLAGS")
 }
 
+func goGcflagsIsEmpty() bool {
+       return "" == os.Getenv("GO_GCFLAGS")
+}
+
 // run runs a test.
 func (t *test) run() {
 	start := time.Now()
@@ -1002,7 +1006,7 @@ func (t *test) run() {
 		useTmp = false
 		var out []byte
 		var err error
-		if len(flags)+len(args) == 0 && goGcflags() == "" && !*linkshared {
+		if len(flags)+len(args) == 0 && goGcflagsIsEmpty() && !*linkshared && goarch == runtime.GOARCH && goos == runtime.GOOS {
 			// If we're not using special go command flags,
 			// skip all the go command machinery.
 			// This avoids any time the go command would