Message 383895 - Python tracker

>_PyInterpreterState_GET():
>   mov    rax,QWORD PTR [rip+0x22a7dd]        # 0x743118 <_PyRuntime+568>
>   mov    rax,QWORD PTR [rax+0x10]

While working on bpo-39465, I wrote PR 20767 to optimize _PyInterpreterState_GET(): single instruction instead of two:

* Add _PyRuntimeState.interp_current member: atomic variable
* _PyThreadState_Swap() sets _PyRuntimeState.interp_current

But I failed to measure any performance difference.