Rollup merge of #149864 - alexcrichton:wasi-avoid-linkat, r=joboet

std: Don't use `linkat` on the `wasm32-wasi*` targets

This commit is a follow-up to rust-lang/rust#147572 and the issue reported at the end of that PR where the `std::fs::hard_link` function is broken after that PR landed. The true culprit and bug here is fixed in WebAssembly/wasi-libc#690 but until that's released in a wasi-sdk version it should be reasonable, on WASI, to skip the `linkat` function.

Author: matthiaskrgr