gh-108987: Fix _thread.start_new_thread() race condition (#109135)

Fix _thread.start_new_thread() race condition. If a thread is created
during Python finalization, the newly spawned thread now exits
immediately instead of trying to access freed memory and lead to a
crash.

thread_run() calls PyEval_AcquireThread() which checks if the thread
must exit. The problem was that tstate was dereferenced earlier in
_PyThreadState_Bind() which leads to a crash most of the time.

Move _PyThreadState_CheckConsistency() from thread_run() to
_PyThreadState_Bind().