Resurrect tunetime, but now as part of get_bit_live().

It should gain a 1% increase in realfreq on my default Raspberry Pi
(700 MHz, clang 3.3, default optimisations) and might help a bit in
sudden scheduler fluctuations induced by other processes.

Since MacOS has no proper translation of CLOCK_MONOTONIC and is marked as
NOLIVE anyway, omit it there.
Ensure that the correct #ifdef logic is used on Linux.