bsd.symver.mk: pass $CFLAGS to $CPP invocation

This allows us to support symbols optionally available based on
configuration, not just on compiler built-in #defines.

Reviewed by: brooks, jrtc27
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45346

(cherry picked from commit aa0bc761d245d2ea1e4b7a0343715cc76859d5da)