tests/tcg: provide a minilib for system tests

tests/tcg: provide a minilib for system tests

We will likely want a few common functions to make up for the fact we
don't have a libc and we don't want to feel like we are programming by
banging rocks together.

I've purloined the printf function from:

  https://git.virtualopensystems.com/dev/tcg_baremetal_tests

Although I have tweaked the names to avoid confusing GCC about clashing
with builtins.

Cc: Alexander Spyridakis <a.spyridakis@virtualopensystems.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>