[RFC] corelow and multi-arch : Move deprecated_add_core_fns to set_gdbarch_regset_from_core_section

[Hui Zhu <teawater@gmail.com>]

Hi,

I got  a some error with multi-arch gdb and a mips core file:

(gdb) core-file ~/tmp/vmcore
warning: "/home/teawater/tmp/vmcore": ambiguous core format, 2 handlers match

warning: wrong size gregset struct in core file
warning: wrong size gregset struct in core file
#0  0x0000000000000000 in ?? ()
(gdb)

This error because sniff_core_bfd return cris_elf_core_fns.   So GDB
use cris_elf_core_fns.fetch_core_registers to analyze this mips core.
It return cris_elf_core_fns because core_sniffer of cris_elf_core_fns
and mips-linux-tdep.c:regset_core_fns is default_core_sniffer.
So I think this is not fit with the multi-arch.

Do you think we can begin move  deprecated_add_core_fns to
set_gdbarch_regset_from_core_section?

BTW, core_vec->core_read_registers have a argument "which" that
gdbarch_regset_from_core_section doesn't have.  Do you think we can
add one to it?

Thanks,
Hui