atomic: Remove cdefs.h includes from atomic_*.h

Avoid including cdefs.h in system headers. Both headers now include
types.h, and we can assume that that pulls in cdefs.h (required for
__typeof usage in some of the atomic macro expansions).

No functional change intended.

Reviewed by: imp, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38039

(cherry picked from commit f7e2f98bffdafff65012fb02ad838c30b7e3502f)