Fixed mutex usage in SharedList and SharedTable. Closes #6988 (#6990)

lib/system/gc.nim

@@ -318,7 +318,7 @@ proc initGC() =
       init(gch.marked)
       init(gch.additionalRoots)
     when hasThreadSupport:
-      gch.toDispose = initSharedList[pointer]()
+      init(gch.toDispose)
 
 when useMarkForDebug or useBackupGc:
   type

lib/system/gc2.nim

@@ -133,7 +133,7 @@ proc initGC() =
     init(gch.additionalRoots)
     init(gch.greyStack)
     when hasThreadSupport:
-      gch.toDispose = initSharedList[pointer]()
+      init(gch.toDispose)
 
 # Which color to use for new objects is tricky: When we're marking,
 # they have to be *white* so that everything is marked that is only

lib/system/gc_ms.nim

@@ -233,7 +233,7 @@ proc initGC() =
       init(gch.allocated)
       init(gch.marked)
     when hasThreadSupport:
-      gch.toDispose = initSharedList[pointer]()
+      init(gch.toDispose)
 
 proc forAllSlotsAux(dest: pointer, n: ptr TNimNode, op: WalkOp) {.benign.} =
   var d = cast[ByteAddress](dest)