Move getcpu.h to 'lib/lttng-ust/'

Move getcpu.h from 'common/ringbuffer' to 'lib/lttng-ust', the
implementation is there and it provides some liblttng-ust symbols to
other components unrelated to the ringbuffer.

* Namespace internal symbols for clarity
* Add comments to the code.

Change-Id: I93da098dc9f90aa6324ad1ac36eaba1c660f0f64
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>