or1k: Correct mask for exception handling

During exception handling we determine the actual exception by using
the program counter which was stored during the preamble at the
exception entry, e.g. 0x844 etc. This is the parameter to the
function. The shifting spared 2 positions to directly use the number *
4 for addressing in the vectors. But to do this properly (see
0x844>>6=0x21 instead of 0x20), we need to mask the lower 8 bits.