Remove use of deprecated_add_core_fns in cris_tdep.c

Remove use of deprecated_add_core_fns in cris_tdep.c

The non-deprecated equivalent is implementing the gdbarch function
iterate_over_regset_sections, this patch does that.

Tested by generating a core file on cris under qemu and comparing
the output of "info registers".

This also fixes this warning when loading cris core files:
  warning: Unexpected size of section `.reg/164' in core file.

gdb/ChangeLog:

2020-02-11  Christian Biesinger  <cbiesinger@google.com>

	* cris-tdep.c (cris_supply_gregset): Change signature to match
	what struct regset expects.
	(cris_regset): New struct.
	(fetch_core_registers): Remove.
	(cris_iterate_over_regset_sections): New function.
	(_initialize_cris_tdep): Don't call deprecated_add_core_fns.
	(cris_gdbarch_init): Call set_gdbarch_iterate_over_regset_sections.

Change-Id: Ieef895b5a2fdc797d1a913cd1c0c07563edfe8e7