Use gdb_bfd_sections in symfile.c

Use gdb_bfd_sections in symfile.c

This changes some functions in symfile.c to avoid
bfd_map_over_sections, in favor of iteration.  Some helper types can
also be removed due to this change.

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

	* symfile.h: (find_lowest_section): Don't declare.
	* symfile.c (find_lowest_section): Now static.  Change
	parameters.
	(struct place_section_arg): Remove.
	(place_section): Change parameters.
	(addr_info_make_relative): Use foreach.
	(symfile_dummy_outputs): Remove.
	(default_symfile_relocate): Use foreach.