STM32: Support board-specific clock configuration

[digitalentity]

I have an STM32F405 board which has an 8 MHz XTAL. The TinyGo has a hardcoded setup for 12 MHz XTAL, which breaks all timings and peripherals (UART, USB). We need to support a few common options (8, 12 and 16 MHz XTAL seem to be relatively common for various STM32Fxxx boards).

We have a wild mix of clock/PLL configs across different STM32 runtime packages. We need to consolidate the M, N, P and Q parameters and have these selectable via a build tag.

[digitalentity]

It's even worse than that... Clock initialization across different hardware targets is done at various places - some in runtime, some in machine, clock selection is also pretty much all over the place.

[digitalentity]

I decided to keep things small. The #5517 only makes the XTAL frequency explicit and owned by the board, rather than implicitly hardcoded. 8, 12 and 16 MHz configurations are supported on STM32 boards.