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

sys/conf/kern.mk

@@ -61,6 +61,9 @@ CWARNEXTRA+=	-Wno-error=misleading-indentation		\
 		-Wno-error=shift-overflow			\
 		-Wno-error=tautological-compare
 .endif
+.if ${COMPILER_VERSION} >= 70100
+CWARNEXTRA+=	-Wno-error=stringop-overflow
+.endif
 .if ${COMPILER_VERSION} >= 70200
 CWARNEXTRA+=	-Wno-error=memset-elt-size
 .endif