Add file name to "Loadable section ... outside of ELF segments" warning

As requested in gdb/23034, I would like to extend the warning message
GDB displays when it detects a loadable section that is outside any
ELF segment.

Before:

$ gdb -q --ex "b systemctl_main" -ex "r" -batch --args systemctl kexec
Breakpoint 1 at 0xc24d: file ../src/systemctl/systemctl.c, line 8752.
warning: Loadable section ".note.gnu.property" outside of ELF segments
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
warning: Loadable section ".note.gnu.property" outside of ELF segments
warning: Loadable section ".note.gnu.property" outside of ELF segments
warning: Loadable section ".note.gnu.property" outside of ELF segments
warning: Loadable section ".note.gnu.property" outside of ELF segments
[snip]

Breakpoint 1, systemctl_main (argv=0x7fffffffd348, argc=2) at ../src/systemctl/systemctl.c:8752
8752                    r = systemctl_main(argc, argv);

After:

warning: Loadable section ".note.gnu.property" outside of ELF segments
  in .gnu_debugdata for /lib64/libcap.so.2
warning: Loadable section ".note.gnu.property" outside of ELF segments
  in .gnu_debugdata for /lib64/libacl.so.1
[snip]

I think this is eminently more useful output.

gdb/ChangeLog
2020-11-13  Keith Seitz  <keiths@redhat.com>

	PR gdb/23034
	* elfread.c (elf_symfile_segments): Output a BFD file name
	for the "Loadable section ... outside of ELF segments" warning.