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/remote.c

@@ -8123,7 +8123,7 @@ Packet: '%s'\n"),
       if (event->ws.kind () != TARGET_WAITKIND_IGNORE)
 	break;
 
-      /* fall through */
+      [[fallthrough]];
     case 'S':		/* Old style status, just signal only.  */
       {
 	int sig;
@@ -8647,7 +8647,7 @@ remote_target::wait_as (ptid_t ptid, target_waitstatus *status,
 	      putpkt (buf);
 	      break;
 	    }
-	  /* fallthrough */
+	  [[fallthrough]];
 	default:
 	  warning (_("Invalid remote reply: %s"), buf);
 	  break;
@@ -10002,7 +10002,7 @@ remote_target::putpkt_binary (const char *buf, int cnt)
 	      return 1;
 	    case '-':
 	      remote_debug_printf_nofunc ("Received Nak");
-	      /* FALLTHROUGH */
+	      [[fallthrough]];
 	    case SERIAL_TIMEOUT:
 	      tcount++;
 	      if (tcount > 3)