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/printcmd.c

@@ -482,7 +482,7 @@ print_scalar_formatted (const gdb_byte *valaddr, struct type *type,
 			       byte_order);
 	  break;
 	}
-      /* FALLTHROUGH */
+      [[fallthrough]];
     case 'f':
       print_floating (valaddr, type, stream);
       break;