Use gdb_bfd_sections in build_section_table

Use gdb_bfd_sections in build_section_table

This changes build_section_table to avoid bfd_map_over_sections, in
favor of iteration.  In this situation it seemed simple to just remove
the helper function entirely.

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

	* exec.c (add_to_section_table): Remove.
	(build_section_table): Use foreach.