[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

[third_party][jitterentropy] Customize for Magenta

Rearrange header files per Magenta conventions, and provide
implementations for some of the basic system interfaces.

Useful implementations for jent_z{alloc,free} are not provided, since we
want more control over how memory is allocated. Note that these two
functions are only called from jent_entropy_collector_{alloc,free}.

To replace the entropy collector alloc/free functions, this commit adds
jent_entropy_collector_init. The new init function initializes an
entropy collector instance using caller-provided memory, and provides
more control over the entropy collector parameters.

Change-Id: Ifab2ee1841c2894cb1a55df1eeabd57cd88298c0