lang/gcc*: disable building of libssp

For today's FreeBSD 14 and earlier, libssp is a nop because nothing on
FreeBSD will be referencing these headers or lib. On FreeBSD 15, it
actively breaks building with FORTIFY_SOURCE enabled and it's not worth
trying to make it work- the potential for GCC improvements comes from
the __builtin_object_size enhancement, not from these headers.

PR: 280265
Approved by: salvadore (maintainer)