Enable limit_rdylib_exports on wasm targets

This commit updates the target specification of wasm targets to set the
`limit_rdylib_exports` value to `true` like it is on other native
platforms. This was originally not implemented long ago as `wasm-ld`
didn't have options for symbol exports, but since then it's grown a
`--export` flag and such to control this. A custom case is needed in the
linker implementation to handle wasm targets as `wasm-ld` doesn't
support linker scripts used on other targets, but other than that the
implementation is straightforward.

The goal of this commit is enable building dynamic libraries on
`wasm32-wasip2` which don't export every single symbol in the Rust
standard library. Currently, without otherwise control over symbol
visibility, all symbols end up being exported which generates
excessively large binaries because `--gc-sections` ends up doing nothing
as it's all exported anyway.

Author: alexcrichton

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -845,6 +845,11 @@ impl<'a> Linker for GccLinker<'a> {
                 self.sess.dcx().emit_fatal(errors::VersionScriptWriteFailure { error });
             }
             self.link_arg("--dynamic-list").link_arg(path);
+        } else if self.sess.target.is_like_wasm {
+            self.link_arg("--no-export-dynamic");
+            for (sym, _) in symbols {
+                self.link_arg("--export").link_arg(sym);
+            }
         } else {
             // Write an LD version script
             let res: io::Result<()> = try {

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

@@ -81,11 +81,6 @@ pub(crate) fn options() -> TargetOptions {
         // threaded model which will legalize atomics to normal operations.
         singlethread: true,
 
-        // Symbol visibility takes care of this for the WebAssembly.
-        // Additionally the only known linker, LLD, doesn't support the script
-        // arguments just yet
-        limit_rdylib_exports: false,
-
         // we use the LLD shipped with the Rust toolchain by default
         linker: Some("rust-lld".into()),
         linker_flavor: LinkerFlavor::WasmLld(Cc::No),