Use gdb_bfd_sections in generic_load

Use gdb_bfd_sections in generic_load

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

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

	* symfile.c (add_section_size_callback): Remove.
	(load_one_section): Rename from load_section_callback.  Change
	parameters.
	(generic_load): Use foreach.