winpthreads: spinlock.c: call `CreateEventW` instead of `CreateEventA`

Unicode APIs such as `CreateEventW` are preffered over ANSI such as
`CreateEventA`; the former, however, are not supported on Win9x systems.

In the current state, winpthreads does not support Win9x systems, so use
Unicode APIs instead.

Signed-off-by: Kirill Makurin <maiddaisuki@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>