Fix mingw atomics (nanomsg#1601)

* Fixes compiling on Windows with Mingw

Fixes the build error: "InterlockedDecrementAcquire64 not defined" on Mingw

* fixes semantics of InterlockedDecrementRelease64 on Mingw

From Microsoft docs, InterlockedDecrementRelease64 returns the resulting decremented value.
The equivalent function on Mingw is `__atomic_sub_fetch`, not `__atomic_fetch_sub` (which returns the previous value).

src/platform/windows/win_thread.c

@@ -26,8 +26,10 @@ static pfnSetThreadDescription set_thread_desc;
  __atomic_add_fetch(a, b, __ATOMIC_RELAXED)
 #define InterlockedIncrementAcquire64(a) \
  __atomic_add_fetch(a, 1, __ATOMIC_ACQUIRE)
+#define InterlockedDecrementAcquire64(a) \
+ __atomic_sub_fetch(a, 1, __ATOMIC_ACQUIRE)
 #define InterlockedDecrementRelease64(a) \
- __atomic_fetch_sub(a, 1, __ATOMIC_RELEASE)
+ __atomic_sub_fetch(a, 1, __ATOMIC_RELEASE)
 #endif
 
 #include <stdlib.h>