Don't link with --export-dynamic on wasm32-wasi

Remove --export-dynamic from the link arguments on the wasm32-wasi
target, as it emits spurious exports and increases code size.

Leave it in place for wasm32-unknown-unknown and
wasm32-unknown-emscripten. Even though it isn't a great solution
there, users are likely depending on its behavior there.

compiler/rustc_target/src/spec/wasm32_unknown_emscripten.rs

@@ -2,6 +2,17 @@ use super::wasm32_base;
 use super::{LinkArgs, LinkerFlavor, PanicStrategy, Target, TargetOptions};
 
 pub fn target() -> Target {
+    let mut options = wasm32_base::options();
+
+    let clang_args = options.pre_link_args.get_mut(&LinkerFlavor::Gcc).unwrap();
+
+    // Rust really needs a way for users to specify exports and imports in
+    // the source code. --export-dynamic isn't the right tool for this job,
+    // however it does have the side effect of automatically exporting a lot
+    // of symbols, which approximates what people want when compiling for
+    // wasm32-unknown-unknown expect, so use it for now.
+    clang_args.push("--export-dynamic".to_string());
+
     let mut post_link_args = LinkArgs::new();
     post_link_args.insert(
         LinkerFlavor::Em,
@@ -28,7 +39,7 @@ pub fn target() -> Target {
         panic_strategy: PanicStrategy::Unwind,
         post_link_args,
         os_family: Some("unix".to_string()),
-        ..wasm32_base::options()
+        ..options
     };
     Target {
         llvm_target: "wasm32-unknown-emscripten".to_string(),