test: skip cross-arch codegen tests in all.bash

The test/codegen tests check all architectures
mentioned in the test file, but this requires
building at least the runtime for that architecture.
This CL changes the test to only check the local
architecture, leaving checking of other architectures
to the relevant builders, as usual.

This cuts 'go run run.go codegen' by 12r 78u 21s.

After this change, all.bash runs in ~4:40 on my laptop.

For #26473.

Change-Id: Ia0354d1aff2df2949f838528c8171410bc42dc8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/177577
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

test/run.go

@@ -34,6 +34,7 @@ var (
 	keep           = flag.Bool("k", false, "keep. keep temporary directory.")
 	numParallel    = flag.Int("n", runtime.NumCPU(), "number of parallel tests to run")
 	summary        = flag.Bool("summary", false, "show summary of results")
+	allCodegen     = flag.Bool("all_codegen", false, "run all goos/goarch for codegen")
 	showSkips      = flag.Bool("show_skips", false, "show skipped tests")
 	runSkips       = flag.Bool("run_skips", false, "run skipped tests (ignore skip and build tags)")
 	linkshared     = flag.Bool("linkshared", false, "")
@@ -653,7 +654,13 @@ func (t *test) run() {
 		// Compile Go file and match the generated assembly
 		// against a set of regexps in comments.
 		ops := t.wantedAsmOpcodes(long)
+		self := runtime.GOOS + "/" + runtime.GOARCH
 		for _, env := range ops.Envs() {
+			// Only run checks relevant to the current GOOS/GOARCH,
+			// to avoid triggering a cross-compile of the runtime.
+			if string(env) != self && !strings.HasPrefix(string(env), self+"/") && !*allCodegen {
+				continue
+			}
 			// -S=2 forces outermost line numbers when disassembling inlined code.
 			cmdline := []string{"build", "-gcflags", "-S=2"}
 			cmdline = append(cmdline, flags...)