gh-146636: abi3t: Define Py_GIL_DISABLED but do not use it (GH-148142)

When compiling for abi3t, define Py_GIL_DISABLED, so that users who
check it to enable additional locking aren't broken.

But also avoid using Py_GIL_DISABLED in Python headers themselves
-- abi3 and abi3t ought to be the same except
the _Py_OPAQUE_PYOBJECT differences.

A check for this is coming in a later PR.
It will require rewriting some preprocessor conditions, some of these
changes are included in this PR.
For _Py_IsOwnedByCurrentThread & supporting functions
I opted to move them to a cpython/ header, as they're rather self-contained.