Avoid thread_local on MacOS to prevent issues with hot reload

[dsnopek]

Fixes #1573

This isn't quite right yet. I think the lock_guard for reload is in the right place, but the one in Wrapped::_set_construct_info() is wrong. We need to lock around the whole memnew() operation, not just the part where we set those variables. I'm not sure the best way to do that yet.

UPDATE: This might be OK now. I need to get a good test.

This depends on PR #1590 because it adds another thread_local, and this way we can address that one too

[dsnopek]

Now that PR #1590 is merged, taking this one out of draft!

[paddy-exe]

Discussed in the GDExtension meeting and tested by user who created the issue with success.

[dsnopek]

Cherry-picked for 4.3 in PR #1695

[ArdaE]

FYI, this PR broke my GDExtension code on macOS, causing the following linker errors:

Linking game.dylib Undefined symbols for architecture arm64: "thread-local wrapper routine for godot::Wrapped::_constructing_extension_class_name", referenced from: ... "thread-local wrapper routine for godot::Wrapped::_constructing_class_binding_callbacks", referenced from: ... ld: symbol(s) not found for architecture arm64 

I've resolved the issue for now by passing the option "use_hot_reload=no" to scons when building godot-cpp.

[dsnopek]

FYI, this PR broke my GDExtension code on macOS, causing the following linker errors:

MacOS builds with hot reload are still working for me. Perhaps doing a scons -c to clean away old build artifacts would help?