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/dcrt0.cc

@@ -551,7 +551,7 @@ get_cygwin_startup_info ()
 	  case _CH_FORK:
 	    in_forkee = true;
 	    should_be_cb = sizeof (child_info_fork);
-	    /* fall through */;
+	    fallthrough;
 	  case _CH_SPAWN:
 	  case _CH_EXEC:
 	    if (!should_be_cb)
@@ -570,7 +570,7 @@ get_cygwin_startup_info ()
 	    break;
 	  default:
 	    system_printf ("unknown exec type %u", res->type);
-	    /* intentionally fall through */
+	    fallthrough;
 	  case _CH_WHOOPS:
 	    res = NULL;
 	    break;