runtime: use rand_hwrng hardwareRand for RP2040/RP2350

[makiuchi-d]

This PR proposes using the existing rand_hwrng.go implementation of hardwareRand() for RP2040/RP2350.

On these targets, hardwareRand() calls machine.GetRNG() on every invocation, so the math/rand global RNG always obtains randomness and does not fall back to fastrand64().

This makes hardwareRand() behave as its name suggests, at the cost of lower performance.

[deadprogram]

Thanks for the improvement here @makiuchi-d and to @eliasnaur for all the help with this set of features.

Now squash/merging.