On CPUs that support the Data Independent Timing feature, enable this feature

On CPUs that support the Data Independent Timing feature, enable this feature
by default in both the kernel and userland.  At this point, this feature has
only been seen on Apple's CPU cores, where turning it on has no measurable
impact on performance.  Turning this feature on should help to mitigate
timing side-channel attacks.

ok deraadt@, beck@