Store objfiles on a std::list

Store objfiles on a std::list

This removes objfile::next and changes objfiles to be stored in a
std::list.

gdb/ChangeLog
2019-12-12  Tom Tromey  <tom@tromey.com>

	* progspace.c (program_space::add_objfile)
	(program_space::remove_objfile): Update.
	(program_space::multi_objfile_p): Remove.
	* objfiles.h (struct objfile) <next>: Remove.
	* objfiles.c (objfile::objfile): Update.
	(put_objfile_before): Update.
	(unlink_objfile): Update.
	* progspace.h (object_files): Remove.
	(struct program_space) <objfiles_head>: Remove.
	<objfiles_list>: New member.
	<objfiles_range, objfiles_safe_range>: Change type.
	(objfiles): Change return type.
	(objfiles_safe): Update.
	(multi_objfile_p): Rewrite and inline.
	(object_files): Remove macro.

Change-Id: Ib4430e3db6f9a390399924379a5c10426c514853