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

[jieyouxu]

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.

  rust/compiler/rustc_target/src/spec/mod.rs

  Lines 2148 to 2153 in f2becdf

  	/// Whether dynamic linking is available on this target. Defaults to false.
  	pub dynamic_linking: bool,
  	/// Whether dynamic linking can export TLS globals. Defaults to true.
  	pub dll_tls_export: bool,
  	/// If dynamic linking is available, whether only cdylibs are supported.
  	pub only_cdylib: bool,

  • This is the case for wasm base, dynamic linking is supported but not dylib crate type, only cdylib is supported.

    rust/compiler/rustc_target/src/spec/base/wasm.rs

    Lines 58 to 62 in f2becdf

    	// we allow dynamic linking, but only cdylibs. Basically we allow a
    	// final library artifact that exports some symbols (a wasm module) but
    	// we don't allow intermediate `dylib` crate types
    	dynamic_linking: true,
    	only_cdylib: true,

Originally posted by @jieyouxu in #130860 (comment)

[jieyouxu]

Related: #132350