Libraries and module types for portable clocks

This library implements portable support for an operating system timesource that is compatible with the MirageOS library interfaces found in: https://github.com/mirage/mirage

It implements an MCLOCK module that represents a monotonic timesource since an arbitrary point, and PCLOCK which counts time since the Unix epoch.

The following sources are used:

• The Unix version uses gettimeofday or clock_gettime, depending on which OS is in use (see clock_stubs.c).
• The freestanding version uses the paravirtual clock source from the hypervisor.