gc/default/default.c: don't call malloc_usable_size when hint is present

Depending on the allocator, `malloc_usable_size` may be very cheap or quite
expensive. On `macOS` for instance, it's about as expensive as `malloc`.

In many case we call `objspace_malloc_size` with as size we initially
requested as `hint`. The real usable size may be a few bytes bigger,
but since we only use that data to feed GC heuristics, I don't think
it's very important to be perfectly accurate.

It would make sense to call `malloc_usable_size` after growing a String
or Array to use the extra capacity, but here we don't do that, so
the call isn't worth its cost.

gc/default/default.c

@@ -7936,10 +7936,11 @@ static inline size_t
 objspace_malloc_size(rb_objspace_t *objspace, void *ptr, size_t hint)
 {
 #ifdef HAVE_MALLOC_USABLE_SIZE
-    return malloc_usable_size(ptr);
-#else
-    return hint;
+    if (!hint) {
+        hint = malloc_usable_size(ptr);
+    }
 #endif
+    return hint;
 }
 
 enum memop_type {