winpthreads: Fix pthread_create_wrapper type

- Make pthread_create_wrapper return an unsigned;

  _beginthreadindex takes a function returning an unsigned. The
  variable holding the return value of pthread_create_wrapper is
  already an unsigned.

- Make pthread_create_wrapper __stdcall.

Co-authored-by: Samuel Hym <samuel.hym@rustyne.lautre.net>
Signed-off-by: Antonin Décimo <antonin@tarides.com>
Signed-off-by: Martin Storsjö <martin@martin.st>