Add some more C functions for the TLS libraries

These are @hannesm's patches to Mini-OS, but moved to mirage-platform.

- 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. They help with the TLS C dependencies.