enable --tlsEmulation:on for --gc:arc (#13685)

* enable --tlsEmulation:on for --gc:arc
* make -d:useMalloc work with --gc:arc --threads:on

lib/system/threads.nim

@@ -79,7 +79,7 @@ when not defined(useNimRtl):
 # use ``stdcall`` since it is mapped to ``noconv`` on UNIX anyway.
 
 type
-  Thread* {.pure, final.}[TArg] = object
+  Thread*[TArg] = object
     core: PGcThread
     sys: SysThread
     when TArg is void:
@@ -98,9 +98,6 @@ proc onThreadDestruction*(handler: proc () {.closure, gcsafe.}) =
   ## A thread is destructed when the ``.thread`` proc returns
   ## normally or when it raises an exception. Note that unhandled exceptions
   ## in a thread nevertheless cause the whole process to die.
-  when not defined(nimNoNilSeqs):
-    if threadDestructionHandlers.isNil:
-      threadDestructionHandlers = @[]
   threadDestructionHandlers.add handler
 
 template afterThreadRuns() =