ssp: add support for _FORTIFY_SOURCE=3

ssp: add support for _FORTIFY_SOURCE=3

If specified, use __builtin_dynamic_object_size() instead of
__builtin_object_size() if supported (GCC 12.0 or later).
This enables buffer overflow checks if the buffer size is non-const
but known during runtime.
Use new macro __ssp_bos_known() instead of the (bos(p) != (size_t)-1)
checks.  The latter is no longer a compile time constant in all cases.
This avoids the generation of unused code.

Signed-off-by: Christian Franke <christian.franke@t-online.de>