gh-116616: Use relaxed atomic ops to access socket module defaulttimeout

[erlend-aasland]

• Issue: socketmodule.c: use atomics to access defaulttimeout in free-threaded build  #116616

[erlend-aasland]

@colesbury, there's also a store in the module exec function. Let me if we need to use atomic access these as well.

[colesbury]

there's also a store in the module exec function...

I don't think we need atomics for the initialization in the module exec function. The module state is not visible to other threads at that point, and the internal import locks provide the necessary synchronization.

[erlend-aasland]

Thanks for the patch and the review, @colesbury 😅