added system.onThreadCreation feature for safe thread local storage initializations

tests/threads/tonthreadcreation.nim

@@ -0,0 +1,22 @@
+discard """
+  output: '''some string here'''
+"""
+
+var
+  someGlobal: string = "some string here"
+  perThread {.threadvar.}: string
+
+proc setPerThread() =
+  {.gcsafe.}:
+    deepCopy(perThread, someGlobal)
+
+proc foo() {.thread.} =
+  echo perThread
+
+proc main =
+  onThreadCreation setPerThread
+  var t: Thread[void]
+  createThread[void](t, foo)
+  t.joinThread()
+
+main()