Add some more C functions for the TLS libraries

- printf and fprintf write to the Mini-OS console, if there is free
  space (note: they return 1 rather than the number of bytes written).
- stdout and stderr are defined (as NULL).
- fflush is provided (now returns 0).
- __stack_chk_guard is defined so stack protection can be used.
- __assert_fail and abort are provided.

These functions are useful because code often uses them in debug macros
and similar.
[xen] Assert that pages passed to the grant share API are page-aligned.

This always happens if they are created via `Io_page.create`, and
probably not true if made by `Cstruct.create`.
Use monotonic time for events

This avoids possible problems when wall-clock time is adjusted.
Merge branch 'monotonic-time' of git://github.com/talex5/mirage-platform

Conflicts:
	CHANGES
Merge branch 'debug-fns' of git://github.com/talex5/mirage-platform

Conflicts:
	CHANGES