Use gdb_bfd_sections in build_objfile_section_table

Use gdb_bfd_sections in build_objfile_section_table

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

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

	* objfiles.c (add_to_objfile_sections): Rename from
	add_to_objfile_sections_full.
	(add_to_objfile_sections): Remove.
	(build_objfile_section_table): Use foreach.