fixes #20516; system.create doesn't work with bitfield objects (#20518)

* Revert "fixes #19000 (#19032)"

This reverts commit 2bda4a30a6.

* fixes #20516; add testcase

tests/stdlib/tsystem.nim

@@ -162,3 +162,10 @@ block:
   reset(y)
   doAssert y.b == {}
 
+block: # bug #20516
+  type Foo = object
+    x {.bitsize:4.}: uint
+    y {.bitsize:4.}: uint
+
+  when not defined(js):
+    let a = create(Foo)