X86: Use the M5PanicFault fault in execute methods instead of calling panic.

X86: Use the M5PanicFault fault in execute methods instead of calling panic.

If an instruction is executed speculatively and hits a situation where it
wants to panic, it should return a fault instead. If the instruction was
misspeculated, the fault can be thrown away. If the instruction wasn't
misspeculated, the fault will be invoked and the panic will still happen.