compiletest: add a proper `supports-crate-type: dylib` directive

Apparently needs-dynamic-linking is not equivalent to checking if dylib or cdylib crate types are supported.

In compiletest, needs-dynamic-linking performs a check based on target cfg's dynamic_linking field + --print=cfg --target $TARGET.
However, target cfg has an additional field only_cdylib which, if dynamic_linking is true, indicates that only cdylib crate type is supported and not dylib. https://github.com/rust-lang/rust/blob/f2becdff0496003217e7fc6fbfcaf2640e162775/compiler/rustc_target/src/spec/mod.rs#L2148-L2153
- This is the case for wasm base, dynamic linking is supported but not dylib crate type, only cdylib is supported. https://github.com/rust-lang/rust/blob/f2becdff0496003217e7fc6fbfcaf2640e162775/compiler/rustc_target/src/spec/base/wasm.rs#L58-L62

rust-lang / rust