Fix cross-builds after 4e5d32a445f90d37966cd6de571978551654e3f3

Add alignment macros to cross-build's sys/cdefs.h

Pull Request: https://github.com/freebsd/freebsd-src/pull/531
MFC after: immediately (build fix)

(cherry picked from commit 94d9439b6be6bd5ef9febfaf38128e0cad91476d)