[Regression] createThread is not GC-safe

[mratsim]

The following compiles in Nim 1.2.0 but fails on latest devel

proc display() {.gcsafe.} =
  echo "Echo from thread ", getThreadId()

proc displayManager() {.gcsafe.} =
  var t: Thread[void]
  t.createThread(display)

proc main() {.gcsafe.} =
  var displayService: Thread[void]
  displayService.createThread(displayManager)

main()

Error:

Error: 'displayManager' is not GC-safe as it calls 'createThread'