gh-113956: Make intern_common thread-safe in free-threaded build (gh-148886)

gh-113956: Make intern_common thread-safe in free-threaded build (gh-148886)

Avoid racing with the owning thread's refcount operations when
immortalizing an interned string: if we don't own it and its refcount
isn't merged, intern a copy we own instead. Use atomic stores in
_Py_SetImmortalUntracked so concurrent atomic reads are race-free.