After a pthread_join() call the handle becomes invalid or could be
recycled by another thread, leading to undefined behavior when
attempting to join it again. Also, on Windows the handle has to be
closed to free it.

Bug b/34883464

Change-Id: Ib20d0539b6b46e331c6378b3a9f0c2a334d34892
Reviewed-on: https://swiftshader-review.googlesource.com/8612Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>