sanitizers: Avoid building genassym.c and genoffset.c with sanitizers

Some, particularly KASAN, may insert redzones around global symbols,
resulting in incorrect offset definitions because genassym.sh (ab)uses
symbol sizes to assign semantic meaning.

(Ideally I would be able to define this pattern in one place, but I
haven't found a way to define a GENSYM_CFLAGS that actually works for
all of the consumers (kern.post.mk, kmod.mk, sys/conf/files*).)

MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: Juniper Networks, Inc.