include: ssp: don't shadow the mempcpy builtin

GCC emits a warning about shadowing a builtin with our mempcpy
declaration, so switch it to using the same model as memcpy() and
use the apparently-existing __builtin___mempcpy_chk().

Reviewed by: kib (earlier version), markj
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D45976