NFC: Feed Pagemap its primitive allocator as template arg

This will let us use Pagemaps further down the dependency stack (specifically,
we're going to want a Pagemap inside the AddressSpaceManager) by letting us
manually tie the knot rather than rely on GlobalVirtual and
default_memory_provider() being defined by the time we want a Pagemap.

src/mem/chunkmap.h

@@ -81,7 +81,7 @@ namespace snmalloc
   using ChunkmapPagemap = std::conditional_t<
     CHUNKMAP_USE_FLATPAGEMAP,
     FlatPagemap<SUPERSLAB_BITS, uint8_t>,
-    Pagemap<SUPERSLAB_BITS, uint8_t, 0>>;
+    Pagemap<SUPERSLAB_BITS, uint8_t, 0, DefaultPrimAlloc>>;
 
   struct ForChunkmap
   {};