Remove "stub_" from the name of generated functions by dynamic link macro

[loloicci]

Macros dynamic_link and callable_point generates stub functions, for example

#[callable_point]
fn add(...) { ... }

generates

extern "C" {
  fn stub_add(...) { ... }
}

This name may conflict with another user-defined function. We can avoid this by generating the following code instead of the stub_* function, which is the same as the entry_point macro.

mod __wasm_export_add {
  extern "C" {
    fn add(...) { ... }
  }
}