cpu: Fix Minor SMT WFI/drain interaction issues

cpu: Fix Minor SMT WFI/drain interaction issues

The behavior of WFI is to cause minor to cease evaluating
pipeline logic until an interrupt is observed, however
a user may wish to drain the system while a core is sleeping
due to a WFI.  This patch makes WFI drain.  If an actual
drain occurs during a WFI, the CPU is already drained and will
immediately be ready for swapping, checkpointing, etc.  This
should not negatively impact performance as WFI instructions
are 'stream-changing' (treated like unpredicted branches), so
all remaining instructions are wrong-path and will be squashed
rapidly.

Change-Id: I63833d5acb53d8dde78f9f0c9611de0ece385e45