Use gdb_bfd_sections in elf_symfile_read

Use gdb_bfd_sections in elf_symfile_read

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

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

	* elfread.c (elf_locate_sections): Change parameters.
	(elf_symfile_read): Use foreach.