machine: compute rp2 clock dividers from crystal and target frequency

[eliasnaur]

Follow-up to #4728 which implemented the algorithm for finding the dividers.

The calculation is computed at compile time by interp, as verified by the unchanging size for example/blinky1 for -target pico.

@soypat you mentioned that you couldn't get TinyGo to compute the dividers at compile time. However, this PR doesn't change the size of the blinky1 example for -target pico1. What did I miss?

[soypat]

Interesting you got the same size. I tried something similar, but instead of using init I tried directly initializing the variable at package level var pllSys, pll... = pllSearch{...}.CalcDiv(...) and I actually got a smaller binary size than the blinky example which told me something was VERY wrong. sure enough when flashing a RP2040 the light would not light up, presumably having crashed/panicked in calculation.

Will try to test it