Change how DWARF index writer finds address map

Change how DWARF index writer finds address map

This changes the DWARF index writer to find the psymtab address map in
the per_bfd object, rather than go via the objfile.

gdb/ChangeLog
2021-03-20  Tom Tromey  <tom@tromey.com>

	* dwarf2/index-write.c (struct addrmap_index_data) <objfile>:
	Remove.
	(add_address_entry): Remove objfile parameter.
	(add_address_entry_worker): Update.
	(write_address_map): Replace objfile parameter with per_bfd.
	(write_gdbindex, write_psymtabs_to_index): Update.