powerpc/booke: Set CPU_FTR_DEBUG_LVL_EXC on 32-bit

Currently 32-bit only cares about this for choice of exception
vector, which is done in core-specific code.  However, KVM will
want to distinguish as well.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>