parisc: Clear stale IIR value on instruction access rights trap

In the Linux kernel, the following vulnerability has been resolved:

parisc: Clear stale IIR value on instruction access rights trap

When a trap 7 (Instruction access rights) occurs, this means the CPU

couldn't execute an instruction due to missing execute permissions on

the memory region. In this case it seems the CPU didn't even fetched

the instruction from memory and thus did not store it in the cr19 (IIR)

register before calling the trap handler. So, the trap handler will find

some random old stale value in cr19.

This patch simply overwrites the stale IIR value with a constant magic

"bad food" value (0xbaadf00d), in the hope people don't start to try to

understand the various random IIR values in trap 7 dumps.