gdb: add type::stub_is_supported / type::set_stub_is_supported

Add the `stub_is_supported` and `set_stub_is_supported` methods on `struct type`, in
order to remove the `TYPE_STUB_SUPPORTED` 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) <stub_is_supported, set_stub_is_supported>: New methods.
	(TYPE_STUB_SUPPORTED): Use type::stub_is_supported, change all write call sites to
	use type::set_stub_is_supported.

Change-Id: I4dfecf2b5df9c2b7bb8db1e9252082140adf3028