[3.12] gh-100227: Lock Around Modification of the Global Allocators State (gh-105516) (gh-105532)

The risk of a race with this state is relatively low, but we play it safe anyway. We do avoid using the lock in performance-sensitive cases where the risk of a race is very, very low.
(cherry picked from commit 68dfa49627)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>

Include/internal/pycore_pymem.h

@@ -18,6 +18,7 @@ typedef struct {
 } debug_alloc_api_t;
 
 struct _pymem_allocators {
+    PyThread_type_lock mutex;
     struct {
         PyMemAllocatorEx raw;
         PyMemAllocatorEx mem;