misc/cgo/test: log error value in testSigprocmask

The test has been flaky, probably due to EAGAIN, but let's find out
for sure.

Updates #25078

Change-Id: I5a5b14bfc52cb43f25f07ca7d207b61ae9d4f944
Reviewed-on: https://go-review.googlesource.com/109359
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

misc/cgo/test/sigprocmask.go

@@ -32,7 +32,7 @@ func IntoGoAndBack() {
 
 func testSigprocmask(t *testing.T) {
 	if r := C.RunSigThread(); r != 0 {
-		t.Error("pthread_create/pthread_join failed")
+		t.Errorf("pthread_create/pthread_join failed: %d", r)
 	}
 	if !blocked {
 		t.Error("Go runtime unblocked SIGIO")