NFC: prepare RemoteCache API for amplification

When post()-ing the RemoteCache to message queues, we push an entire bucket
onto a remote allocator's incoming queue (specifically, the allocator owning
the front Remote in the bucket we're moving).  In order to do that, we need to
exceed the bounds of the Remote allocation and reference its Allocslab header
(to get the ->message_queue).  On StrictProvenance architectures, this will
require that we amplify the head Remote* and then engage in some pointer math.

While Remotes contain the address of the message_queue as the allocator's
identity, this may not be a pointer, just an address, and may have undergone
obfuscation anyway.

src/mem/alloc.h

@@ -561,8 +561,10 @@ namespace snmalloc
         l->last = r;
       }
 
-      void post(alloc_id_t id)
+      void post(LargeAlloc<MemoryProvider>* large_allocator, alloc_id_t id)
       {
+        UNUSED(large_allocator);
+
         // When the cache gets big, post lists to their target allocators.
         capacity = REMOTE_CACHE;
 
@@ -896,7 +898,7 @@ namespace snmalloc
         return;
 
       stats().remote_post();
-      remote.post(get_trunc_id());
+      remote.post(&large_allocator, get_trunc_id());
     }
 
     /**
@@ -1565,7 +1567,7 @@ namespace snmalloc
       remote.dealloc(target->trunc_id(), offseted, sizeclass);
 
       stats().remote_post();
-      remote.post(get_trunc_id());
+      remote.post(&large_allocator, get_trunc_id());
     }
 
     ChunkMap& chunkmap()

src/mem/globalalloc.h

@@ -126,7 +126,7 @@ namespace snmalloc
           if (alloc->remote.capacity < REMOTE_CACHE)
           {
             alloc->stats().remote_post();
-            alloc->remote.post(alloc->get_trunc_id());
+            alloc->remote.post(&alloc->large_allocator, alloc->get_trunc_id());
             done = false;
           }