[PATCH] fix ARCH_HAS_PREFETCH

include/linux/prefetch.h does a strange thing: if the arch doesn't have
the prefectch functions, this header defines no-op version of them and
then defines ARCH_HAS_PREFETCH.  So there's no way for mainline code to
know if the architecture *really* has prefetch instructions.

This information loss is unfortunate.  Examples:

	for (i = 0; i < N; i++)
		prefetch(foo[i]);

   Problem is, if `prefetch' is a no-op, the compiler will still
   generate an empty busy-wait loop.  Which it must do.  We need to
   know the truth about ARCH_HAS_PREFETCH to correctly elide that loop.

include/linux/prefetch.h

@@ -39,17 +39,14 @@
  */
  
 #ifndef ARCH_HAS_PREFETCH
-#define ARCH_HAS_PREFETCH
 static inline void prefetch(const void *x) {;}
 #endif
 
 #ifndef ARCH_HAS_PREFETCHW
-#define ARCH_HAS_PREFETCHW
 static inline void prefetchw(const void *x) {;}
 #endif
 
 #ifndef ARCH_HAS_SPINLOCK_PREFETCH
-#define ARCH_HAS_SPINLOCK_PREFETCH
 #define spin_lock_prefetch(x) prefetchw(x)
 #endif