Convert objfiles.c to type-safe registry API

Convert objfiles.c to type-safe registry API

This changes objfiles.c to use the type-safe registry API.

gdb/ChangeLog
2019-05-08  Tom Tromey  <tom@tromey.com>

	* objfiles.c (objfile_pspace_info): Add destructor and
	initializers.
	(objfiles_pspace_data): Change type.
	(~objfile_pspace_info): Rename from objfiles_pspace_data_cleanup.
	(get_objfile_pspace_data): Update.
	(objfiles_bfd_data): Change type.
	(get_objfile_bfd_data): Update.
	(objfile_bfd_data_free, _initialize_objfiles): Remove.