Rollup merge of #115548 - Zoxc:parallel-extract, r=wesleywiser

Extract parallel operations in `rustc_data_structures::sync` into a new `parallel` submodule

This extracts parallel operations in `rustc_data_structures::sync` into a new `parallel` submodule. This cuts down on the size of the large `cfg_if!` in `sync` and makes it easier to compare between serial and parallel variants.

Author: matthiaskrgr