bring back code that uses export and fix tests

partial revert of 1fdebc1dc4

example/shared_library/mathtest.zig

@@ -1,6 +1,3 @@
-comptime {
-    @export("add", add);
-}
-extern fn add(a: i32, b: i32) -> i32 {
+export fn add(a: i32, b: i32) -> i32 {
     a + b
 }