[SimpleJIT] When finalizing multiple functions, make them all executable at the end. (#474)

Add `publish()` function to cranelift-module's `Backend` trait, which
allows `finalize_all()` to defer making memory executable until it
has finished all of the patching it needs to do.

lib/simplejit/src/backend.rs

@@ -335,8 +335,6 @@ impl<'simple_jit_backend> Backend for SimpleJITBackend {
             }
         }
 
-        // Now that we're done patching, make the memory executable.
-        self.code_memory.set_executable();
         func.code
     }
 
@@ -397,10 +395,15 @@ impl<'simple_jit_backend> Backend for SimpleJITBackend {
             }
         }
 
-        self.readonly_memory.set_readonly();
         (data.storage, data.size)
     }
 
+    fn publish(&mut self) {
+        // Now that we're done patching, prepare the memory for execution!
+        self.readonly_memory.set_readonly();
+        self.code_memory.set_executable();
+    }
+
     /// SimpleJIT emits code and data into memory as it processes them, so it
     /// doesn't need to provide anything after the `Module` is complete.
     fn finish(self) -> () {}