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

[emaste]

Summary

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

[emaste]

@jrtc27 wondered if cpp accepts all of the same flags as cc. I imagine so but we could also limit this to ${CFLAGS:M-D*} perhaps.

[brooks]

Probably worth testing with a gcc toolchain to see if we need to filter. Including -D* might be a place to start, but some other options to impact defines in ways that might impact symbols (e.g., -march, -mabi, -mcpu) so it might be we want to have a deny list (certainly we could filter out -Wl*.)

[emaste]

it might be we want to have a deny list (certainly we could filter out -Wl*.)

Yeah, I could start with `:N-Wl* as there is no linking so no reason to pass linker flags.