gdb: add type::target_is_stub / type::set_target_is_stub

gdb: add type::target_is_stub / type::set_target_is_stub

Add the `target_is_stub` and `set_target_is_stub` methods on `struct
type`, in order to remove the `TYPE_TARGET_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) <target_is_stub, set_target_is_stub>:
	New methods.
	(TYPE_TARGET_STUB): Use type::is_stub, change all write call
	sites to use type::set_target_is_stub.

Change-Id: I9c71a89adc7ae8d018db9ee156f41c623be0484a