gh-111964: Implement stop-the-world pauses (gh-112471)

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:

* `_PyEval_StopTheWorldAll()` and `_PyEval_StartTheWorldAll()` (for the global runtime)
* `_PyEval_StopTheWorld()` and `_PyEval_StartTheWorld()` (per-interpreter)

(The function names may change.)

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

Include/internal/pycore_runtime_init.h

@@ -116,6 +116,9 @@ extern PyTypeObject _PyExc_MemoryError;
         }, \
         .faulthandler = _faulthandler_runtime_state_INIT, \
         .tracemalloc = _tracemalloc_runtime_state_INIT, \
+        .stoptheworld = { \
+            .is_global = 1, \
+        }, \
         .float_state = { \
             .float_format = _py_float_format_unknown, \
             .double_format = _py_float_format_unknown, \