LinuxKPI: add counted_by() and nonstring

Add checks for counted_by(_x) and nonstring as and depending
on compiler support enable the attribute/feature.

This is needed to make mtk76 and ath1xk drivers compile.

Sposnored by: The FreeBSD Foundation
Reviewed by: emaste, dim
Differential Revision: https://reviews.freebsd.org/D44587

(cherry picked from commit 2a8c50592f7fce8bceedf004f674a5a9eae0fd46)