Bring back the magic emulator-detect instruction, but with an enable bit.

In order to set the enable bit, call
wait4(0x456d756c,0x61746f72,0x4d616769,0x633a2d29), a call I hope is
unlikely enough to occur by accident.  (Those four numbers are ASCII
for "Emul", "ator", "Magi", and "c:-)".)

Without the enable bit, the emulator-detect xnor magic is suppressed.