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.