Enable guard pages in CHECK_CLIENT

Change the behaviour to use PROT_NONE for reservations in CHECK_CLIENT
mode.  This means that we only provide access once data is actually
being used.

src/mem/corealloc.h

@@ -146,7 +146,7 @@ namespace snmalloc
       FreeListBuilder<false> b;
       SNMALLOC_ASSERT(b.empty());
 
-#ifdef CHECK_CLIENT
+#ifdef SNMALLOC_CHECK_CLIENT
       // Structure to represent the temporary list elements
       struct PreAllocObject
       {
@@ -214,7 +214,7 @@ namespace snmalloc
       meta->free_queue.close(fl, key);
       void* p = finish_alloc_no_zero(fl.take(key), sizeclass);
 
-#ifdef CHECK_CLIENT
+#ifdef SNMALLOC_CHECK_CLIENT
       // Check free list is well-formed on platforms with
       // integers as pointers.
       size_t count = 1; // Already taken one above.