gh-129984: Mark immortal objects as deferred

[DinoV]

Marks immortal objects as having the deferred reference counting bit, allowing these objects to be cached in the specializing interpreter loop.

Gets a significant perf win on deltablue where small ints are stored in a class and loaded repeatedly: https://github.com/facebookexperimental/free-threading-benchmarking/blob/main/results/bm-20250210-3.14.0a4%2B-8e67151-NOGIL/bm-20250210-vultr-x86_64-DinoV-immortal_deferred-3.14.0a4%2B-8e67151-vs-base.svg

• Issue: Immortal objects don't have the deferred bit set by default #129984

[mpage]

I think we probably don't want to mark immortal objects as deferred and instead should allow immortal objects to be stored in inline caches (i.e. check that the object either uses deferred refcounting or is immortal)

[colesbury]

I think this makes sense. When we are checking if an object uses deferred reference counting, we nearly always want to include immortal objects too. We can rename the flag if it improves clarity.

• We'd probably want to set the bit in _Py_SetImmortalUntracked too.
• We can simplify the check in

  cpython/Include/internal/pycore_stackref.h

  Line 222 in 1feaecc

  if (_Py_IsImmortal(obj) || _PyObject_HasDeferredRefcount(obj)) {

  if we're consistent about it.

[colesbury]

LGTM