The --disable-gil builds occasionally need to pause all but one thread. Some examples include:

• Cyclic garbage collection, where this is often called a "stop the world event"
• Before calling fork(), to ensure a consistent state for internal data structures
• During interpreter shutdown, to ensure that daemon threads aren't accessing Python objects

This adds the following functions to implement global and per-interpreter pauses:

• _PyRuntimeState_StopTheWorld and _PyRuntimeState_StartTheWorld
• _PyInterpreterState_StopTheWorld and _PyInterpreterState_StartTheWorld

These functions are no-ops outside of the --disable-gil build.

This also adds _PyRWMutex, a "readers-writer" lock, which is used to serialize global stop-the-world pauses with per-interpreter pauses.

• Issue: Implement stop-the-world functionality (for --disable-gil builds) #111964