[3.12] GH-110455: Guard `assert(tstate->thread_id > 0)` with `GH-ifndef HAVE_PTHREAD_STUBS` (GH-110487) (GH-110491)

GH-110455: Guard `assert(tstate->thread_id > 0)` with `GH-ifndef HAVE_PTHREAD_STUBS` (GH-110487)
(cherry picked from commit 5fd8821cf8eb1fe2e8575f8c7cc747cf78855a88)

Co-authored-by: Brett Cannon <brett@python.org>