Use gdb_bfd_sections in ELF osabi tag sniffing

Use gdb_bfd_sections in ELF osabi tag sniffing

This changes some ELF osabi tag-sniffing functions to avoid
bfd_map_over_sections, in favor of iteration.  I could only readily
test the generic one.

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

	* osabi.h (generic_elf_osabi_sniff_abi_tag_sections): Update.
	* osabi.c (generic_elf_osabi_sniff_abi_tag_sections): Change
	parameters.
	(generic_elf_osabi_sniffer): Use foreach.
	* mips-sde-tdep.c (mips_sde_elf_osabi_sniffer): Use foreach.
	* arm-tdep.c (arm_elf_osabi_sniffer): Use foreach.