Pass --export-dynamic to LLD for wasm

This should handle recent symbol visibility changes happening, although
we'll likely want to tweak this in the future!

Author: alexcrichton

src/librustc_codegen_ssa/back/linker.rs

@@ -1037,6 +1037,12 @@ impl<'a> Linker for WasmLd<'a> {
         // indicative of bugs, let's prevent them.
         self.cmd.arg("--fatal-warnings");
 
+        // The symbol visibility story is a bit in flux right now with LLD.
+        // It's... not entirely clear to me what's going on, but this looks to
+        // make everything work when `export_symbols` isn't otherwise called for
+        // things like executables.
+        self.cmd.arg("--export-dynamic");
+
         // LLD only implements C++-like demangling, which doesn't match our own
         // mangling scheme. Tell LLD to not demangle anything and leave it up to
         // us to demangle these symbols later.