Clear Cause.BD after we use instruction_pointer_set to override
EPC.

This can prevent exception_epc check against instruction code at
new return address.
It won't be considered as "in delay slot" after epc being overridden
anyway.
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>