[POWERPC] Don't try to just continue if xmon has no input device

Currently, if xmon has no input device (as is generally the case on
G5 powermacs), and we drop into xmon as a result of a fatal exception,
it will return 1, which die() interprets as "continue without causing
an oops".  This fixes it by making xmon() return 0 in the case where
it has no input device.

Signed-off-by: Paul Mackerras <paulus@samba.org>