gh-128137: Update PyASCIIObject to handle interned field with the atomic operation

[corona10]

• Issue: Race in PyUnicode_InternFromString under free-threading #128137

[corona10]

@colesbury I am not sure what you intended.
To maintain the original size, I squeeze the state field from 32 bits to 16 bits and reserve 16 bits for the interned field.

[corona10]

f30b355 breaks WASI test: https://github.com/python/cpython/actions/runs/12469477403/job/34802702354?pr=128196 (object size is increased).

[colesbury]

I think this remain in the state struct. It's okay for a struct to contain both non-bitfield and bitfield members:

• It avoids a potential unnecessary breakage from moving the field
• Keeping it in state will make it easier to keep state 32-bits due to alignment.

[colesbury]

I think this should have a NEWS entry given that we're changing the size of a semi-public field.

[corona10]

Since we touch the semi-public field, I am not sure about backporting this PR.
Let's just leave 3.13t as buggy status because it is just the experimental build.

[colesbury]

LGTM, assuming the tests pass

[colesbury]

I agree -- I don't think we should backport this.

[corona10]

There looks like issue with Win32, I will see tomorrow :)

[vstinner]

cc @methane

[corona10]

I am now debugging on my Windows machine..

[corona10]

@colesbury @methane

I 've updated test_str.py and test_sys.py for win32 and WASI, but there seems to be an alignment issue that we didn't care about before.
Do you think the current implementation is fine, or would you prefer to redesign the layout to be more consistent?
(Not sure about this currently)