Fix return value of sys_arch_mbox_tryfetch() in Win32 port

It should be SYS_MBOX_EMPTY, not SYS_ARCH_TIMEOUT.
SYS_MBOX_EMPTY is defined to SYS_ARCH_TIMEOUT, so there is no bug.
But for good readability of the code,  SYS_MBOX_EMPTY should be used.

contrib/ports/win32/sys_arch.c

@@ -688,7 +688,7 @@ sys_arch_mbox_tryfetch(sys_mbox_t *q, void **msg)
       *msg  = NULL;
     }
 
-    return SYS_ARCH_TIMEOUT;
+    return SYS_MBOX_EMPTY;
   }
 }