systm.h: Include cdefs.h first

Ever since r143063, machine/atomic.h requires cdefs.h. So, include it
first. Weak support: style(9) tells us to include cdefs.h first.

Argument against: since code that includes systm.h still compiles,
compilation units that include systm.h must already include cdefs.h. So, an
argument could be made that the cdefs.h include could just be removed
entirely. That is maybe a bigger change and not one I am interested in
bikeshedding.

Universe compiles.

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