winpthreads: Check if requested stack size fits in an unsigned int

The requested stack size is a `size_t`, whereas _beginthreadex takes
an `unsigned int`. This provides a bit of input sanitation, and
prevents a compiler warning.

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>