bsd.sys.mk: Add NO_WDANGLING_POINTER helper variable.
This variable expands to -Wno-dangling-pointer on GCC 12+.
Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D37532
(cherry picked from commit ad96424e31a8c8c87fac8077fd4304c6fb133e25)