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/amd64-netbsd-nat.c

@@ -55,9 +55,7 @@ static int amd64nbsd32_r_reg_offset[] =
 
 static amd64_bsd_nat_target<nbsd_nat_target> the_amd64_nbsd_nat_target;
 
-void _initialize_amd64nbsd_nat ();
-void
-_initialize_amd64nbsd_nat ()
+INIT_GDB_FILE (amd64nbsd_nat)
 {
   amd64_native_gregset32_reg_offset = amd64nbsd32_r_reg_offset;
   amd64_native_gregset32_num_regs = ARRAY_SIZE (amd64nbsd32_r_reg_offset);