Also do RSB refilling when context switching, after vmexits, and

Also do RSB refilling when context switching, after vmexits, and
when vmlaunch or vmresume fails.

Follow the lead of clang and the intel recommendation and do an lfence
after the pause in the speculation-stop path for retpoline, RSB refill,
and meltover ASM bits.

ok kettenis@ deraadt@