st/nine: Introduce nine_context

st/nine: Introduce nine_context

nine_context is a new structure which goal will be
to contain all internal states. It will be the states
of the second thread in the to-be-introduced CSMT mode.

This patch moves several internal states to nine_context,
while the next patches add the other fields.

Signed-off-by: Axel Davy <axel.davy@ens.fr>