gh-101659: Isolate "obmalloc" State to Each Interpreter (gh-101660)

This is strictly about moving the "obmalloc" runtime state from
`_PyRuntimeState` to `PyInterpreterState`.  Doing so improves isolation
between interpreters, specifically most of the memory (incl. objects)
allocated for each interpreter's use.  This is important for a
per-interpreter GIL, but such isolation is valuable even without it.

FWIW, a per-interpreter obmalloc is the proverbial
canary-in-the-coalmine when it comes to the isolation of objects between
interpreters.  Any object that leaks (unintentionally) to another
interpreter is highly likely to cause a crash (on debug builds at
least).  That's a useful thing to know, relative to interpreter
isolation.

Include/internal/pycore_obmalloc_init.h

@@ -54,9 +54,13 @@ extern "C" {
 #  error "NB_SMALL_SIZE_CLASSES should be less than 64"
 #endif
 
-#define _obmalloc_state_INIT(obmalloc) \
+#define _obmalloc_global_state_INIT \
     { \
         .dump_debug_stats = -1, \
+    }
+
+#define _obmalloc_state_INIT(obmalloc) \
+    { \
         .pools = { \
             .used = _obmalloc_pools_INIT(obmalloc.pools), \
         }, \