[jitterentropy] Workaround lack of timer on x86

On x86, the system timer isn't calibrated until after LK_INIT_LEVEL_VM
(so that we can use HPET or APIC for calibration, if needed). To
workaround this, use TSC directly, as long as the device has an
invariant TSC.

Change-Id: Idd13d606c73f69b05eb962c5295c2f4eb808d816