Set crt_static_allow_dylibs to true for Emscripten target

And add a test. This is followup work to PR 151704. It introduced a regression where
cargo is now unwilling to build cdylibs for Emscripten because `crt_static_default` is
`true` but `crt_static_allows_dylibs` is `false`. Unfortunately the added test does not
fail without the change because the validation logic is in Cargo, not in rustc. But it's
good to have some coverage of this anyways.

compiler/rustc_target/src/spec/targets/wasm32_unknown_emscripten.rs

@@ -19,8 +19,15 @@ pub(crate) fn target() -> Target {
         pre_link_args,
         post_link_args,
         relocation_model: RelocModel::Pic,
+        // crt_static should always be true for an executable and always false
+        // for a shared library. There is no easy way to indicate this and it
+        // doesn't seem to matter much so we set crt_static_allows_dylibs to
+        // true and leave crt_static as true when linking dynamic libraries.
+        // wasi also sets crt_static_allows_dylibs: true so this is at least
+        // aligned between wasm targets.
         crt_static_respected: true,
         crt_static_default: true,
+        crt_static_allows_dylibs: true,
         panic_strategy: PanicStrategy::Unwind,
         no_default_libraries: false,
         families: cvs!["unix", "wasm"],

src/tools/compiletest/src/directives/directive_names.rs

@@ -249,6 +249,7 @@
     "only-unix",
     "only-visionos",
     "only-wasm32",
+    "only-wasm32-unknown-emscripten",
     "only-wasm32-unknown-unknown",
     "only-wasm32-wasip1",
     "only-watchos",

tests/run-make/wasm-emscripten-cdylib/foo.rs

@@ -0,0 +1,4 @@
+#[no_mangle]
+pub extern "C" fn foo() -> i32 {
+    42
+}

tests/run-make/wasm-emscripten-cdylib/rmake.rs

@@ -0,0 +1,25 @@
+//! Check that cdylib crate type is supported for the wasm32-unknown-emscripten
+//! target and produces a valid Emscripten dynamic library.
+
+//@ only-wasm32-unknown-emscripten
+
+use run_make_support::{bare_rustc, rfs, wasmparser};
+
+fn main() {
+    bare_rustc().input("foo.rs").target("wasm32-unknown-emscripten").crate_type("cdylib").run();
+
+    // Verify the output is a valid wasm file with a dylink.0 section
+    let file = rfs::read("foo.wasm");
+    let mut has_dylink = false;
+
+    for payload in wasmparser::Parser::new(0).parse_all(&file) {
+        let payload = payload.unwrap();
+        if let wasmparser::Payload::CustomSection(s) = payload {
+            if s.name() == "dylink.0" {
+                has_dylink = true;
+            }
+        }
+    }
+
+    assert!(has_dylink, "expected dylink.0 section in emscripten cdylib output");
+}