Rollup merge of #124159 - joboet:move_pal_thread_parking, r=ChrisDenton

Move thread parking to `sys::sync`

Part of #117276.

I'll leave the platform-specific API abstractions in `sys::pal`, as per the initial proposal. I'm not entirely sure whether we'll want to keep it that way, but that remains to be seen.

r? ``@ChrisDenton`` (if you have time)

Author: matthiaskrgr