Limit symbols exported from proc macros

Only __rustc_proc_macro_decls_*__ and rust_metadata_* need to be
exported for proc macros to work. All other symbols only increase binary
size and have the potential to conflict with symbols from the host
compiler.

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -656,9 +656,7 @@ impl<'a> Linker for GccLinker<'a> {
             return;
         }
 
-        if crate_type == CrateType::ProcMacro {
-            return;
-        }
+        // FIXME hide #[no_mangle] symbols for proc-macros
 
         let is_windows = self.sess.target.is_like_windows;
         let path = tmpdir.join(if is_windows { "list.def" } else { "list" });