[POWERPC] Fix stolen time for SMT without LPAR

For POWERPC, stolen time accounts for cycles lost to the hypervisor or
PURR cycles attributed to the other SMT thread.  Hence, when a PURR is
available, we should still calculate stolen time, irrespective of being
virtualised.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>