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/codegen/init-large-type.rs

@@ -7,8 +7,7 @@
 // optimisation.
 
 //@ pretty-expanded FIXME #23616
-//@ ignore-emscripten no threads support
-
+//@ needs-threads
 #![feature(intrinsics)]
 
 use std::{mem, thread};

tests/ui/codegen/issue-28950.rs

@@ -1,5 +1,5 @@
 //@ run-pass
-//@ ignore-emscripten no threads
+//@ needs-threads
 //@ compile-flags: -O
 
 // Tests that the `vec!` macro does not overflow the stack when it is