fix atomics memory ordering for sharedptr

Consume was used in the original pr nim-lang/Nim#10485, haven't questioned before nim-lang/Nim#19212

Author: planetis-m

Diff for: threading/smartptrs.nim

@@ -81,15 +81,13 @@ type
 
 proc `=destroy`*[T](p: var SharedPtr[T]) =
   if p.val != nil:
-    if p.val[].counter.load(Consume) == 0:
+    if fetchSub(p.val[].counter, 1, AcqRel) == 0:
       `=destroy`(p.val[])
       deallocShared(p.val)
-    else:
-      atomicDec(p.val[].counter)
 
 proc `=copy`*[T](dest: var SharedPtr[T], src: SharedPtr[T]) =
   if src.val != nil:
-    atomicInc(src.val[].counter)
+    discard fetchAdd(src.val[].counter, 1, Relaxed)
   if dest.val != nil:
     `=destroy`(dest)
   dest.val = src.val