Enable exception traces by default so that early user space breakage
(e.g. broken code in initrd) can be easily indentified.

If not needed afterwards it can be disabled by writing '0' in one of
these two files:

/proc/sys/kernel/userprocess_debug
/proc/sys/debug/exception-trace
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>