atomic(9): Implement atomic_testand(clear|set)_ptr

For current architectures, these are just aliases for the existing
operation on the relevant scalar integer.

Reviewed by: imp, kib
Obtained from: CheriBSD
Sponsored by: AFRL, DARPA
Differential Revision: https://reviews.freebsd.org/D47631

(cherry picked from commit a80b9ee15aa0f2da9f9f0c48e13324e0e5f2e840)