Add a type definition for the cyclic timer callback function.
Add a type definition for the cyclic timer callback function.

The cyclic timer is a high-resolution timer allows timeouts at nanosecond
intervals where hardware support is available. Typically on i386 there
is no HPET (high performance event timer) like the one Intel started
specifying some time in 2004, so the best that tye cyclic timer subsystem
can do is run at Hz.

The cyclic timer code itself is ported from OpenSolaris and is covered
by the CDDL, so it is only loaded as a module. This function type definition
is used in machine-dependent code to provide a hook for the module to
register it's callback function.

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