Various fixes for OE (#418)

* Add default for getting chunk allocator state

Makes the API same between the two configurations.

* Reduce address space usage for Open Enclave

* Fix OE Pal concept

* Add support for Pal not to provide time.

The lazy return of pages to the OS uses a simple time
based heuristic.  This enables a PAL to not support time,
and return the memory to a central pool immediately.

* Update src/backend/backend.h

Co-authored-by: Amaury Chamayou <amaury@xargs.fr>

Co-authored-by: Amaury Chamayou <amaury@xargs.fr>

src/backend/fixedglobalconfig.h

@@ -25,7 +25,7 @@ namespace snmalloc
 
   public:
     static ChunkAllocatorState&
-    get_chunk_allocator_state(typename Backend::LocalState*)
+    get_chunk_allocator_state(typename Backend::LocalState* = nullptr)
     {
       return chunk_allocator_state;
     }