missing check against overlay_debugging in objfiles.c

missing check against overlay_debugging in objfiles.c

This fixes a problem where the debugger is trying to locate a minimal
symbol from its address, when the symbol is inside a section whose
VMA is different from its LMA.

We have a program that was built on ppc-elf using a linker script
such that data sections are placed in ROM, and then loaded onto RAM
at execution.  So their VMA addresses are indeed different from
their LMA address.

Unfortunately, there is one place where GDB gets slightly confused
into thinking that these data sections are overlayed, while it's
not the case here.  This show up when trying to print the list of
Ada tasks, where GDB is unable to determine their names, and thus
prints a generic `Ravenscar task' instead:

    (gdb) info tasks
       ID       TID P-ID Pri State                  Name
        1     1d580      127 Delay Sleep            Ravenscar task
        2     183f8      127 Delay Sleep            Ravenscar task
    *   3     13268      127 Runnable               Ravenscar task

We expected:

    (gdb) info tasks
       ID       TID P-ID Pri State                  Name
        1     1d580      127 Delay Sleep            environment_task
        2     183f8      127 Delay Sleep            raven2
    *   3     13268      127 Runnable               raven1

The name of the task is determined by looking up the symbol table
using the task ID, which is the address where the symbol is defined.
So, ada-tasks calls...

    msym = lookup_minimal_symbol_by_pc (task_id);

... which in turn first tries to determine the section associated
to this address (find_pc_section), which itself uses a map of sections
to find it. The map itself is recomputed every time objfiles are
loaded/changed by `update_section_map'. And `update_section_map'
relies on `insert_section_p' to determine whether the section should
be inserted in the map or not.

This is where things get interesting for us, because `insert_section_p'
simply rejects overlay sections:

  if (lma != 0 && lma != bfd_section_vma (abfd, section)
      && (bfd_get_file_flags (abfd) & BFD_IN_MEMORY) == 0)
    /* This is an overlay section.  IN_MEMORY check is needed to avoid
       discarding sections from the "system supplied DSO" (aka vdso)
       on some Linux systems (e.g. Fedora 11).  */
    return 0;

However, it shouldn't reject our section in this case, since overlay
debugging is off.  The fix is to add a check that overlay debugging
is active before rejecting the section.  This is similar to what is
done in `section_is_overlay' (which takes obj_section objects), for
instance.

gdb/Changelog:

        * objfiles.c (insert_section_p): Do not detect overlay sections
        if overlay debugging is off.