[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 68dfa496278aa21585eb4654d5f7ef13ef76cb50)

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