[SPARC64]: Kill all %pstate changes in context switch code.

They are totally unnecessary because:

1) Interrupts are already disabled when switch_to()
   runs.

2) We don't use hard-coded alternate globals any longer.

This found a case in rtrap, which still assumed alternate
global %g6 was current_thread_info(), and that is fixed
by this changeset as well.

Signed-off-by: David S. Miller <davem@davemloft.net>