random: Define a macro for getting the CPU cycle count

[markj]

Summary

Entropy queue entries always include the low 32 bits of a CPU cycle
count reading. Introduce a macro for this instead of hard-coding
get_cyclecount() calls everywhere; this is handy for testing purposes
since this way, random(4)'s use of the cycle counter (e.g., the number
of bits we use) can be changed in one place.

No functional change intended.

[cem]

lgtm otherwise

sys/dev/random/random_harvestq.h

46–47

Could this just be an inline function instead of a macro?

[markj]

sys/dev/random/random_harvestq.h

46–47

It could, I didn't do it that way since it would require pulling in machine/cpu.h whereas only two files actually need the macro. But, random_harvestq.h isn't widely included anyway, so it should be fine to have it be a function.

[markj]

Switch to using a function. (Will update the commit message too.)

[cem]

Thanks!