Retpolines are an anti-pattern for IBT, so we need to shift protecting

Retpolines are an anti-pattern for IBT, so we need to shift protecting
userspace from cross-process BTI to the kernel.  Have each CPU track
the last pmap run on in userspace and the last vmm VCPU in guest-mode
and use the IBPB msr to flush predictors right before running in
userspace on a different pmap or entering guest-mode on a different
VCPU.  Codepatch-nop the userspace bits and conditionalize the vmm
bits to keep working if IBPB isn't supported.

ok deraadt@ kettenis@