libc: move __ssp_overlap back out into <ssp/ssp.h>

We'll be using it in some upcoming definitions in headers, so move it
back now but slap a warning on it. Our upcoming uses will all be inside
of inline functions, so we're not overly concerned about double
evaluation immediately.

Reviewed by: markj
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D45677