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/dwarf2/index-write.c

@@ -1768,9 +1768,7 @@ gdb_index ()
 } /* selftests namespace.  */
 #endif
 
-void _initialize_dwarf_index_write ();
-void
-_initialize_dwarf_index_write ()
+INIT_GDB_FILE (dwarf_index_write)
 {
 #if GDB_SELF_TEST
   selftests::register_test ("gdb_index", selftests::gdb_index);