Apply changes from commitid FWrfGfO9Ojnsh1mq to libkern.

Apply changes from commitid FWrfGfO9Ojnsh1mq to libkern.
From original commit message: To mitigate against spectre attacks,
AMD processors without the IBRS feature need an lfence instruction
after every near ret.  See software techniques for managing speculation
on AMD processors revision 9.17.20 mitigation G-5.

ok deraadt@