Use gdb_bfd_sections in core_target_open

Use gdb_bfd_sections in core_target_open

This changes core_target_open to avoid bfd_map_over_sections, in favor
of iteration.

gdb/ChangeLog
2020-09-19  Tom Tromey  <tom@tromey.com>

	* corelow.c (add_to_thread_list): Change parameters.
	(core_target_open): Use foreach.