atomic: Fix and cleanup SDL_UnlockSpinlock()

- Add missing SDL_MemoryBarrierRelease() in the generic codepath
- Remove MSVC x86/x64 case which is now identical to the generic codepath
- Fix Solaris barrier to ensure prior stores are visible before unlocking

(cherry picked from commit 95ae0c194ec785d5a7ecf8b83a34f7f2ae29b07a)

src/atomic/SDL_spinlock.c

@@ -171,16 +171,13 @@ void SDL_UnlockSpinlock(SDL_SpinLock *lock)
     SDL_COMPILE_TIME_ASSERT(locksize, sizeof(*lock) == sizeof(long));
     _InterlockedExchange_rel((long *)lock, 0);
 
-#elif defined(_MSC_VER)
-    _ReadWriteBarrier();
-    *lock = 0;
-
 #elif defined(SDL_PLATFORM_SOLARIS)
     // Used for Solaris when not using gcc.
-    *lock = 0;
     membar_producer();
+    *lock = 0;
 
 #else
+    SDL_MemoryBarrierRelease();
     *lock = 0;
 #endif
 }