winpthreads: use `mbsrtowcs` instead of `mbstowcs`

`pthread_setname_np` function defined in thread.c uses `mbstowcs` function
which is not thread-safe. Use `mbsrtowcs` instead.

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