atomic: Fix infinite recursion in SDL_CompilerBarrier() fallback

On some platforms, SDL_MemoryBarrierRelease() is defined to
SDL_CompilerBarrier(). If SDL_CompilerBarrier() is also defined to
the fallback spinlock acquire/release, then we will infinitely
recurse in SDL_UnlockSpinlock(). Avoid this by not unlocking the
temporary spinlock we create.

Author: cgutman

include/SDL_atomic.h

@@ -148,8 +148,9 @@ void _ReadWriteBarrier(void);
 extern __inline void SDL_CompilerBarrier(void);
 #pragma aux SDL_CompilerBarrier = "" parm [] modify exact [];
 #else
+/* We don't unlock here to avoid possible infinite recursion */
 #define SDL_CompilerBarrier()   \
-{ SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
+{ SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); }
 #endif
 
 /**