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.

tests/ui/threads-sendsync/spawning-with-debug.rs

@@ -3,7 +3,7 @@
 #![allow(unused_mut)]
 //@ ignore-windows
 //@ exec-env:RUST_LOG=debug
-//@ ignore-emscripten no threads support
+//@ needs-threads
 
 // regression test for issue #10405, make sure we don't call println! too soon.