os/signal: increase deliver time for signal testcase

This increases the time to wait for signals to be delivered in the
    TestAtomicStop testcase. When running gccgo tests on ppc64 or ppc64le,
    there are intermittent failures in this test because the wait time is
    too small.
    
    Updates golang/go#29046
    
    Reviewed-on: https://go-review.googlesource.com/c/153879

From-SVN: r267068

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-4db15776701a7cfc1abcef08f7a98c63343d17e6
+be6dbbdbe29f39d6d2ec0d5574ced398cb6e250a
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/os/signal/signal_test.go

@@ -432,12 +432,12 @@ func atomicStopTestProgram() {
 
 		// At this point we should either die from SIGINT or
 		// get a notification on cs. If neither happens, we
-		// dropped the signal. Give it a second to deliver,
-		// which is far far longer than it should require.
+		// dropped the signal. It is given 2 seconds to
+		// deliver, as needed for gccgo on some loaded test systems.
 
 		select {
 		case <-cs:
-		case <-time.After(1 * time.Second):
+		case <-time.After(2 * time.Second):
 			if !printed {
 				fmt.Print("lost signal on tries:")
 				printed = true