gdb: add type::is_stub / type::set_is_stub

gdb: add type::is_stub / type::set_is_stub

Add the `is_stub` and `set_is_stub` methods on `struct type`, in order
to remove the `TYPE_STUB` macro.  In this patch, the macro is changed to
use the getter, so all the call sites of the macro that are used as a
setter are changed to use the setter method directly.  The next patch
will remove the macro completely.

gdb/ChangeLog:

	* gdbtypes.h (struct type) <is_stub, set_is_stub>: New methods.
	(TYPE_STUB): Use type::is_stub, change all write call sites to
	use type::set_is_stub.

Change-Id: Ie935e8fe72c908afd8718411e83f4ff00c386bf3