arm: add missing atomic-pointer functions

The pointer function types were missing for these functions, so add
them. Valuable for places where type sizes vary by architecture.

Differential Revision: https://reviews.freebsd.org/D37778
Reviewed by: imp, andrew
Pull Request: https://github.com/freebsd/freebsd-src/pull/1126

(cherry picked from commit 98c276811aece750cd93cec53cd6c8f926519da5)