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.

Tools/c-analyzer/cpython/ignored.tsv

@@ -309,6 +309,7 @@ Objects/obmalloc.c	-	_PyMem	-
 Objects/obmalloc.c	-	_PyMem_Debug	-
 Objects/obmalloc.c	-	_PyMem_Raw	-
 Objects/obmalloc.c	-	_PyObject	-
+Objects/obmalloc.c	-	last_final_leaks	-
 Objects/obmalloc.c	-	usedpools	-
 Objects/typeobject.c	-	name_op	-
 Objects/typeobject.c	-	slotdefs	-