Bug 1949155 - Use Relaxed memory ordering for malloc_initialized. r=glandium

We read this flag very often, so we want those reads to be fastest possible. We already protect writes by gInitLock, and on MSVC we even trust it to work without Atomic.
So Relaxed should be safe here.

Differential Revision: https://phabricator.services.mozilla.com/D238973

memory/build/mozjemalloc.cpp

@@ -684,7 +684,11 @@ static void* base_alloc(size_t aSize);
 // threads are created.
 static bool malloc_initialized;
 #else
-static Atomic<bool, MemoryOrdering::ReleaseAcquire> malloc_initialized;
+// We can rely on Relaxed here because this variable is only ever set when
+// holding gInitLock. A thread that still sees it false while another sets it
+// true will enter the same lock, synchronize with the former and check the
+// flag again under the lock.
+static Atomic<bool, MemoryOrdering::Relaxed> malloc_initialized;
 #endif
 
 // This lock must be held while bootstrapping us.