gdb: add type::is_gnu_ifunc / type::set_is_gnu_ifunc

Add the `is_gnu_ifunc` and `set_is_gnu_ifunc` methods on `struct type`, in
order to remove the `TYPE_GNU_IFUNC` 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_gnu_ifunc, set_is_gnu_ifunc>: New methods.
	(TYPE_GNU_IFUNC): Use type::is_gnu_ifunc, change all write call sites to
	use type::set_is_gnu_ifunc.

Change-Id: Ic23ba8c5b8e589d9fc368385111aa16a94e014e2