fix #12726 Cannot take the compile-time sizeof Atomic types (#15928)

* fix #12726 Cannot take the compile-time sizeof Atomic types

* fix for arch 32

tests/stdlib/concurrency/tatomics_size.nim

@@ -0,0 +1,18 @@
+discard """
+  targets: "c cpp"
+"""
+import std/atomics
+
+block testSize: # issue 12726
+  type
+    Node = ptr object
+      # works
+      next: Atomic[pointer]
+      f:AtomicFlag
+    MyChannel = object
+      # type not defined completely
+      back: Atomic[ptr int]
+      f: AtomicFlag
+  static:
+    doAssert sizeof(Node) == sizeof(pointer)
+    doAssert sizeof(MyChannel) == sizeof(pointer) * 2