misc/cgo/testcarchive: increase timeout duration in TestOsSignal

This test is slightly flaky on the s390x builder and I suspect that
the 100ms timeout is a little too optimistic when the VM is starved.
Increase the timeout to 5s to match the other part of the test.

Fixes #26231.

Change-Id: Ia6572035fb3efb98749f2c37527d250a4c779477
Reviewed-on: https://go-review.googlesource.com/122315
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testcarchive/src/libgo3/libgo3.go

@@ -29,13 +29,13 @@ func ResetSIGIO() {
 	signal.Reset(syscall.SIGIO)
 }
 
-// SawSIGIO returns whether we saw a SIGIO within a brief pause.
+// SawSIGIO reports whether we saw a SIGIO.
 //export SawSIGIO
 func SawSIGIO() C.int {
 	select {
 	case <-sigioChan:
 		return 1
-	case <-time.After(100 * time.Millisecond):
+	case <-time.After(5 * time.Second):
 		return 0
 	}
 }