Testsuite: Ensure interrupt-daemon-attach doesn't run forever

Looking at the AArch64 buildbot, I noticed about two dozen old instances of
interrupt-daemon-attach taking up a full 100% cpu each.

If the test fails then the test binary relies on an alarm to ensure it dies
after 60 seconds.

As per the Linux man page for alarm:
  Alarms created by alarm() ... are not inherited by children created via fork.

Update the test to add an alarm in the child and also put a sleep in the
child loop so it does not constantly consume cpu.

Note I haven't managed to re-create why the test failed.  This fix will just
stop it hanging and consuming cpu when it does.

gdb/testsuite/ChangeLog:

	* gdb.base/interrupt-daemon-attach.c (main): Add alarm and sleep
	in child.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2019-03-27  Alan Hayward  <alan.hayward@arm.com>
+
+	* gdb.base/interrupt-daemon-attach.c (main): Add alarm and sleep
+	in child.
+
 2019-03-26  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* gdb.python/py-prettyprint.c (struct container) <is_map_p>: New

gdb/testsuite/gdb.base/interrupt-daemon-attach.c

@@ -43,6 +43,7 @@ main ()
       break;
 
     default:
+      /* In parent process.  */
       while (1)
 	{
 	  marker ();
@@ -50,12 +51,18 @@ main ()
 	}
     }
 
+  /* In child process.  */
+
+  /* Alarms are not inherited by child processes. Set the alarm again to stop
+     the test case running forever.  */
+  alarm (60);
+
   /* Detach from controlling terminal.  */
   if (setsid () == (pid_t) -1)
     return 1;
 
-  for (;;)
-    ;
+  while (1)
+    usleep (1000);
 
   return 0;
 }