Cygwin: Add 'fallthrough' pseudo keyword for switch/case use

This patch has been inspired by the Linux kernel patch

  294f69e662d1 compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use

written by Joe Perches <joe AT perches DOT com> based on an idea from
Dan Carpenter <dan DOT carpenter AT oracle DOT com>.  The following text
is from the original log message:

Reserve the pseudo keyword 'fallthrough' for the ability to convert the
various case block /* fallthrough */ style comments to appear to be an
actual reserved word with the same gcc case block missing fallthrough
warning capability.

All switch/case blocks now should end in one of:

	break;
	fallthrough;
	goto <label>;
	return [expression];
	continue;

In C mode, GCC supports the __fallthrough__ attribute since 7.1,
the same time the warning and the comment parsing were introduced.

Cygwin-only: add an explicit -Wimplicit-fallthrough=5 to the build
flags.

winsup/cygwin/select.cc

@@ -469,7 +469,7 @@ was_timeout:
 	  pthread::static_cancel_self ();
 	  /*NOTREACHED*/
 	}
-      /*FALLTHRU*/
+      fallthrough;
     default:
       /* Timer event? */
       if (wait_ret == timer_idx)
@@ -1679,7 +1679,7 @@ thread_socket (void *arg)
 	    case WAIT_OBJECT_0:
 	      if (!i)	/* Socket event set. */
 		goto out;
-	      /*FALLTHRU*/
+	      fallthrough;
 	    default:
 	      break;
 	    }