gh-116167: Allow disabling the GIL with PYTHON_GIL=0 or -X gil=0 (#116338)

In free-threaded builds, running with `PYTHON_GIL=0` will now disable the
GIL. Follow-up issues track work to re-enable the GIL when loading an
incompatible extension, and to disable the GIL by default.

In order to support re-enabling the GIL at runtime, all GIL-related data
structures are initialized as usual, and disabling the GIL simply sets a flag
that causes `take_gil()` and `drop_gil()` to return early.

Lib/test/test_embed.py

@@ -523,6 +523,8 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
         CONFIG_COMPAT['_pystats'] = 0
     if support.Py_DEBUG:
         CONFIG_COMPAT['run_presite'] = None
+    if support.Py_GIL_DISABLED:
+        CONFIG_COMPAT['enable_gil'] = -1
     if MS_WINDOWS:
         CONFIG_COMPAT.update({
             'legacy_windows_stdio': 0,