kern.mk: -Wno-error=stringop-overflow for gcc due to false positives

Demote gcc's Wstringop-overflow to Wno-error due to false positives.
E.g. the riscv64 build with gcc 8.3.0 has been failing with this warning
since r355062 [1]. A bug has been filed with gcc [2]. The warning was
first introduced in gcc 7.1 [3]. Hopefully we can avoiding suppressing
the warning in future gcc versions.

[1] https://ci.freebsd.org/job/FreeBSD-head-riscv64-build/16691/
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92718
[3] https://gcc.gnu.org/wiki/WarningHistory

Reviewed by: markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D22603