random: mix in cycle counter when jitter timer fires

Rather than just relying on interaction between cache lines of the timer
and the main loop, also explicitly take into account the fact that the
timer might fire at some time that's hard to predict, due to scheduling,
interrupts, or cross-CPU conditions. Mix in a cycle counter during the
firing of the timer, in addition to the existing one during the
scheduling of the timer. It can't hurt and can only help.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>