bsd.sys.mk: Add NO_WZERO_LENGTH_BOUNDS helper variable.

This variable expands to -Wno-zero-length-bounds on GCC 10+.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D37537

(cherry picked from commit 9134d9265c54a02257292c32b2144c401a86678f)