Use C++17 [[fallthrough]] attribute

This changes gdb to use the C++17 [[fallthrough]] attribute rather
than special comments.

This was mostly done by script, but I neglected a few spellings and so
also fixed it up by hand.

I suspect this fixes the bug mentioned below, by switching to a
standard approach that, presumably, clang supports.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=23159
Approved-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Luis Machado <luis.machado@arm.com>
Approved-By: Pedro Alves <pedro@palves.net>

gdb/fbsd-nat.c

@@ -1641,7 +1641,7 @@ fbsd_nat_target::stop_process (inferior *inf)
       if (status.sig () == GDB_SIGNAL_STOP)
 	break;
 
-      /* FALLTHROUGH */
+      [[fallthrough]];
     default:
       /* Some other event has occurred.  Save the current
 	 event.  */