libc: switch to __weak_symbol for fortified interpose stubs

GCC doesn't like #pragma weak macro(foo), but that's fine; just slap
a __weak_symbol on the definition to more reliably make it weak
anyways.

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