compiletest: Add a `//@ needs-threads` directive
This commit is extracted from #122036 and adds a new directive to the `compiletest` test runner, `//@ needs-threads`. This is intended to capture the need that a target must implement threading to execute a specific test, typically one that uses `std::thread`. This is primarily done for WebAssembly targets which currently do not have threads by default. This enables transitioning a lot of `//@ ignore-wasm*`-style ignores into a more self-documenting `//@ needs-threads` directive. Additionally the `wasm32-wasi-preview1-threads` target, for example, does actually have threads, but isn't tested in CI at this time. This change enables running these tests for that target, but not other wasm targets.
