[POWERPC] print backtrace when entering xmon

xmon does not print a backtrace per default. This is bad on systems with
USB keyboard, the most needed info about the crash is lost.
print a backtrace during the very first xmon entry.

Booting with xmon=nobt disables the autobacktrace functionality.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>