Improve MemoryBarrier() implementation

See thread "[HarfBuzz] compilation error of 0.9.26 with MinGW"
started by Werner.

src/hb-atomic-private.hh

@@ -47,18 +47,22 @@
 #define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 
-#if defined(__MINGW32__) && !defined(MemoryBarrier)
+/* MinGW has a convoluted history of supporting MemoryBarrier
+ * properly.  As such, define a function to wrap the whole
+ * thing. */
 static inline void _HBMemoryBarrier (void) {
+#if !defined(MemoryBarrier)
   long dummy = 0;
   InterlockedExchange (&dummy, 1);
-}
-# define MemoryBarrier _HBMemoryBarrier
+#else
+  MemoryBarrier ();
 #endif
+}
 
 typedef LONG hb_atomic_int_t;
 #define hb_atomic_int_add(AI, V)	InterlockedExchangeAdd (&(AI), (V))
 
-#define hb_atomic_ptr_get(P)		(MemoryBarrier (), (void *) *(P))
+#define hb_atomic_ptr_get(P)		(_HBMemoryBarrier (), (void *) *(P))
 #define hb_atomic_ptr_cmpexch(P,O,N)	(InterlockedCompareExchangePointer ((void **) (P), (void *) (N), (void *) (O)) == (void *) (O))