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
MFC after: 3 days
Reviewed by: emaste, dim
Differential Revision: https://reviews.freebsd.org/D44587