Fix build issue with const and volatile and the

myriad ways that the various compliers treat this. The
only safe prefetch appears to be for AMD. The other
compilers either are not volatile or are not const :(

Reported by:	Michael Tuexen

sys/sys/kern_prefetch.h

@@ -34,7 +34,7 @@ kern_prefetch(const volatile void *addr, void* before)
 #if defined(__amd64__)
 	__asm __volatile("prefetcht1 (%1)":"=rm"(*((int32_t *)before)):"r"(addr):);
 #else
-	__builtin_prefetch(addr);
+/*	__builtin_prefetch(addr);*/
 #endif
 }