[POWERPC] Set lower flag bits in regs->trap to indicate debug level exception

We use the low bits of regs->trap as flag bits.  We already indicate
critical and machine check level exceptions via this mechanism.  Extend it
to indicate debug level exceptions.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>