s390/smp: perform cpu reset before delegating work to target cpu

Clear CPU state (e.g. all TLB entries, prefetched instructions, etc.)
of the target CPU, however without clearing register contents before
starting any work on it.

This puts the target CPU in a more defined state compared to the
current Stop + Restart sigp orders.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>