Add TOC calculation for XCOFF binary in AIX and remove legacy line number information.

Add TOC calculation for XCOFF binary in AIX and remove legacy line number information.

Co-authored-by: Simon Marchi <simon.marchi@polymtl.ca>
Closes https://sourceware.org/bugzilla/show_bug.cgi?id=33606

This is a patch to bring back certain XCOFF reading sections back to the GDB code which was removed during the STABS removal.
This patch also removes the legacy line table calculation for STABS since we no longer will support it.

The issue we removed code that will get us the TOC offset in AIX.
This will now cause regressions.

For example,Consider a code where we create a simple library x as below.

int g_in_lib = 777;

int lib_func() {

        return g_in_lib + 1;

}

int lib_func();

Then we use this library X  in main().

int main() {

        printf ("lib_func() = %d \n", lib_func());

        return 0;

}

If we as of today compile master branch in AIX and try to call lib_func() from GDB we get,

GNU gdb (GDB) 18.0.50.20251112-git
Breakpoint 1, main () at //gdb_tests/main.c:5
5           printf ("lib_func() = %d \n", lib_func());

(gdb) call lib_func()

$1 = 536875277

(gdb) q

which is a garbage value instead of 778.

DWARF will not have any information about TOC to maintain uniformity with other operating system.

TOC (Table Of Contents) is a part of XCOFF/AIX ABI and is required for:
1: Loading shared libraries as we need TOC that contain pointers to access global variables and functions entry points.
2: Function calls like the above call where AIX expects register r2 = pointer to TOC which gives fast access to global data plus an ofset
3: Large code model = TOC solves the fact that PPC64 can't embed large 64 bit addresses.

So we need to get GDB to fetch this even though we only read DWARF debug sections in the XCOFF binary. (AIX uses GCC-13 now which produces only DWARF now.).

In the above case since the toc_offset is not there now we cannot access lib_func() causing the regression.

The patch right now brings back the code required to fetch the same. Once this patch is applied we get,

GNU gdb (GDB) 18.0.50.20251204-git
Copyright (C) 2025 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "powerpc64-ibm-aix7.2.0.0".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.

+------------------------------------------------------------------------------+
| Find the GDB manual online at:                                               |
| http://www.gnu.org/software/gdb/documentation/.                              |
| For help, type "help".                                                       |
| Type "apropos <word>" to search for commands related to <word>               |
+------------------------------------------------------------------------------+
..
Reading symbols from //gdb_tests/main...
(gdb) b main
Breakpoint 1 at 0x10000530: file //gdb_tests/main.c, line 5.
(gdb) r
Starting program: /gdb_tests/main

Breakpoint 1, main () at //gdb_tests/main.c:5
5           printf ("lib_func() = %d \n", lib_func());
(gdb) call lib_func()
$1 = 778
(gdb) q
A debugging session is active.

        Inferior 1 [process 7340312] will be killed.

Quit anyway? (y or n) y

Also some clean ups of code and additions, they are:
1: Replaced old APIs like bfd_map_over_sections with gdb_bfd_sections() and range-based loops.
2: Used helpers like obstack_strndup instead of manual allocation like changing p = (char *) obstack_alloc (&objfile->objfile_obstack, and strncpy (p, symbol->n_name, E_SYMNMLEN);
to *name = obstack_strndup(&objfile->objfile_obstack, symbol->n_name, E_SYMNMLEN);
3: Removed unused macros as unnecessary global variables as you mentioned
4: Replaced perror_with_name with error() and bfd_errmsg. See: error(_("reading symbol table: %s"), bfd_errmsg(bfd_get_error()));
5: Also used bfd_get_section_alloc_size().
6: Eliminated the xcoff_find_targ_sec_arg struct used in GDB 17 or earlier because it is no longer necessary for context handling.
7: Eliminated the find_targ_sec () used in GDB 17 or earlier since we find the bfd_sect in xcoff_secnum_to_sections().

Approved-By: Tom Tromey <tom@tromey.com>