ssp: switch to _limits.h for the upper bound of size_t

The definitions in _stdint.h has some complications around visibility
that _limits.h does not have. Switch to __SIZE_T_MAX to avoid those.

This fixes the devel/gperf, devel/glib20 and math/mpfr builds with
_FORTIFY_SOURCE enabled to unlock a large fraction of the ports tree to
build.

Reported by: Shawn Webb (HardenedBSD)
Sponsored by: Klara, Inc.
Sponsored by: Stormshield