compiletest: Add a //@ needs-threads directive

This commit is extracted from rust-lang#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.

Author: alexcrichton

tests/ui/threads-sendsync/mpsc_stress.rs

@@ -1,6 +1,6 @@
 //@ run-pass
 //@ compile-flags:--test
-//@ ignore-emscripten
+//@ needs-threads
 
 use std::sync::mpsc::channel;
 use std::sync::mpsc::TryRecvError;

tests/ui/threads-sendsync/send-resource.rs

@@ -4,7 +4,7 @@
 #![allow(non_camel_case_types)]
 
 //@ pretty-expanded FIXME #23616
-//@ ignore-emscripten no threads support
+//@ needs-threads
 
 use std::thread;
 use std::sync::mpsc::channel;