Change file initialization to use INIT_GDB_FILE macro

This patch introduces a new macro, INIT_GDB_FILE.  This is used to
replace the current "_initialize_" idiom when introducing a per-file
initialization function.  That is, rather than write:

    void _initialize_something ();
    void
    _initialize_something ()
    {
       ...
    }

... now you would write:

    INIT_GDB_FILE (something)
    {
       ...
    }

The macro handles both the declaration and definition of the function.

The point of this approach is that it makes it harder to accidentally
cause an initializer to be omitted; see commit 2711e475 ("Ensure
cooked_index_entry self-tests are run").  Specifically, the regexp now
used by make-init-c seems harder to trick.

New in v2: un-did some erroneous changes made by the script.

The bulk of this patch was written by script.
Regression tested on x86-64 Fedora 41.

gdb/corelow.c

@@ -2157,9 +2157,7 @@ core_target_find_mapped_file (const char *filename,
   return targ->lookup_mapped_file_info (filename, addr);
 }
 
-void _initialize_corelow ();
-void
-_initialize_corelow ()
+INIT_GDB_FILE (corelow)
 {
   add_target (core_target_info, core_target_open,
 	      filename_maybe_quoted_completer);