target-openrisc: Correct wrong epcr register in interrupt handler

This patch corrects several misbehaviors during an interrupt process.
Most of the time the pc is already correct and therefore no special treatment
of the exceptions is necessary.

Tested by checking crashing programs which otherwise work in or1ksim.

Signed-off-by: Sebastian Macke <sebastian@macke.de>
Reviewed-by: Jia Liu <proljc@gmail.com>
Signed-off-by: Jia Liu <proljc@gmail.com>
