runtime: use CompareAndSwap to avoid racy issue while two threads crash concurrently & fix typo

Author: zzkcode

src/runtime/runtime-gdb_unix_test.go

@@ -361,7 +361,7 @@ func TestGdbCoreCrashThreadBacktrace(t *testing.T) {
 	err = cmd.Wait()
 	t.Logf("child output:\n%s", output.String())
 	if err == nil {
-		t.Fatalf("Wait succeeded, want SIGSEGV")
+		t.Fatalf("Wait succeeded, want SIGABRT")
 	}
 	ee, ok := err.(*exec.ExitError)
 	if !ok {

src/runtime/signal_unix.go

@@ -753,10 +753,11 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 
 	if docrash {
 		isCrashThread := false
-		if crashing.Load() == 0 {
+		if crashing.CompareAndSwap(0, 1) {
 			isCrashThread = true
+		} else {
+			crashing.Add(1)
 		}
-		crashing.Add(1)
 		if crashing.Load() < mcount()-int32(extraMLength.Load()) {
 			// There are other m's that need to dump their stacks.
 			// Relay SIGQUIT to the next m by sending it to the current process.