Immortal objects don't have the deferred bit set by default

[DinoV]

Bug report

Bug description:

_PyObject_HEAD_INIT defines objects w/ an immortal reference count, but it doesn't set _PyGC_BITS_DEFERRED. This results in these objects (like small ints) not being cachable in the specializing interpreter loop for free threaded builds.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-129984: Mark immortal objects as deferred #129985