gdb: rename type::{arch,objfile} -> type::{arch_owner,objfile_owner}

I think this makes the names of the methods clearer, especially for the
arch.  The type::arch method (which gets the arch owner, or NULL if the
type is not arch owned) is easily confused with the get_type_arch method
(which returns an arch no matter what).  The name "arch_owner" will make
it intuitive that the method returns NULL if the type is not arch-owned.

Also, this frees the type::arch name, so we will be able to morph the
get_type_arch function into the type::arch method.

gdb/ChangeLog:

	* gdbtypes.h (struct type) <arch>: Rename to...
	<arch_owner>: ... this, update all users.
	<objfile>: Rename to...
	<objfile_owner>: ... this, update all users.

Change-Id: Ie7c28684c7b565adec05a7619c418c69429bd8c0