Avoid using to_string() on ExternalName to create strings for symbol names.

ExternalName's `to_string()` prepends a '%' for Cretonne's text syntax,
but for creating symbol names we just want the raw bytes.

lib/obj/src/emit_module.rs

@@ -2,6 +2,8 @@ use cretonne::settings;
 use cretonne::settings::Configurable;
 use faerie::Artifact;
 use wasmstandalone_runtime;
+use std::error::Error;
+use std::str;
 
 /// Emits a module that has been emitted with the `WasmRuntime` runtime
 /// implementation to a native object file.
@@ -24,11 +26,13 @@ pub fn emit_module<'module>(
     for (i, function_relocs) in relocations.iter().enumerate() {
         assert!(function_relocs.is_empty(), "relocations not supported yet");
         let body = &compilation.functions[i];
+        let external_name =
+            wasmstandalone_runtime::get_func_name(compilation.module.imported_funcs.len() + i);
+        let string_name = str::from_utf8(x.as_ref()).map_err(|err| {
+            err.description().to_string()
+        })?;
 
-        obj.add_code(
-            wasmstandalone_runtime::get_func_name(compilation.module.imported_funcs.len() + i),
-            body.clone(),
-        );
+        obj.add_code(string_name, body.clone());
     }
 
     Ok(())