src/comp.c: Use `pending_funcalls` to fix bug#64494

src/comp.c: Use `pending_funcalls` to fix bug#64494

Make sure `comp.el` is never loaded synchronously by simply
delaying all calls to `native--compile-async` via `pending_funcalls`.

* lisp/startup.el (comp--compilable, comp--delayed-sources): Don't declare.
(startup--require-comp-safely)
(startup--honor-delayed-native-compilations): Delete functions.
(normal-top-level): Don't call `startup--honor-delayed-native-compilations`.

* src/comp.c (maybe_defer_native_compilation): Use `pending_funcalls`.
(syms_of_comp): Delete `Vcomp__delayed_sources` and `comp__compilable`.
Define `Qnative__compile_async`.