Add hello world program

This commit adds an implementation
of hello world for the Cell BEA. This
has been implemented to learn how
to execute code on the SPEs using
the libspe2. The code also demonstrates
basics of interaction with the spufs.
hello: Improve error handling

This commit improves the handling
of errors by using likely/unlikely
macros to advise the compiler to
optimise for the expected branch
outcome.
hello: Replace sleep() by messaging

Instead of sleeping for a second to
ensure that the PPU is done reading
the spufs entries it is better to have
the PPU communicate to the SPUs using
the mailbox mechanism.