Rollup merge of #158252 - hkBst:std-os-cfg-select, r=jhpratt

Use `cfg_select` in `std::os`

https://github.com/rust-lang/rust/pull/81969 replaced `cgf_if` with bare `cfg`, but since `cfg_select` is now built in, it shouldn't pose a problem to rust-analyzer any more, so we should be able to go back to better code.