atomic: Make atomic_san.h self-contained

This file assumes that uint*_t and __typeof() are defined, so ensure
that consumers don't have to pull in the requisite includes themselves.
This is consistent with sys/atomic_common.h.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D37939

(cherry picked from commit 6ebd8466af2a33d88f491799287035aa0477a5c8)