bsd.sys.mk: Add NO_WUSE_AFTER_FREE helper variable.

This variable expands to -Wno-use-after-free on GCC 12+.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D36817

(cherry picked from commit 67b0751249183749c302f37e114481f8b9550032)