SPI library code erroneously assumed that:
- abs() is a C function, so include stdlib.h is required.
  what happens instead is Arduino.h shadows `abs()` with it's own macro
- uint32_t() - int32_t() promotes to int32_t, thus needing abs()

Fix both issues, leaving existing calculations as-is.

- restore abs call, cast freq to correctly display the intent
- update magic numbers comments
- move some spiclk_t magic numbers to func consts