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`.
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
release 2.0.1
Merge branch 'profile' of git://github.com/talex5/mirage-platform

Conflicts:
	CHANGES
	xen/_vars
resolve merge conflict in CHANGES
Get OCaml headers from the ocaml directory

Before, most headers were duplicated in the include directory (and some
important onces were missing).
Install a pkg-config file for mirage-xen

This allows programs compiling C stubs to find the mirage-xen header
files. It also includes the dependencies on openlibm and Mini-OS.

The Libs: entry is not currently used, but in future the mirage tool
may prefer to use this rather than specifying the linker options
manually.