xen: x86 & generic: change to __builtin_prefetch()

Quoting Andi Kleen in Linux b483570a13be from 2007:
    gcc 3.2+ supports __builtin_prefetch, so it's possible to use it on all
    architectures. Change the generic fallback in linux/prefetch.h to use it
    instead of noping it out. gcc should do the right thing when the
    architecture doesn't support prefetching

    Undefine the x86-64 inline assembler version and use the fallback.

ARM wants to use the builtins.

Fix a pair of spelling errors, one of which was from Lucas De Marchi in the
Linux tree.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Acked-by: Tim Deegan <tim@xen.org>

xen/include/xen/prefetch.h

@@ -28,24 +28,17 @@
 	prefetchw(x)	- prefetches the cacheline at "x" for write
 	spin_lock_prefetch(x) - prefectches the spinlock *x for taking
 	
-	there is also PREFETCH_STRIDE which is the architecure-prefered 
+	there is also PREFETCH_STRIDE which is the architecture-preferred
 	"lookahead" size for prefetching streamed operations.
 	
 */
 
-/*
- *	These cannot be do{}while(0) macros. See the mental gymnastics in
- *	the loop macro.
- */
- 
 #ifndef ARCH_HAS_PREFETCH
-#define ARCH_HAS_PREFETCH
-static inline void prefetch(const void *x) {;}
+#define prefetch(x) __builtin_prefetch(x)
 #endif
 
 #ifndef ARCH_HAS_PREFETCHW
-#define ARCH_HAS_PREFETCHW
-static inline void prefetchw(const void *x) {;}
+#define prefetchw(x) __builtin_prefetch(x,1)
 #endif
 
 #ifndef ARCH_HAS_SPINLOCK_PREFETCH