[PATCH] x86_64 : prefetchw() can fall back to prefetch() if !3DNOW

If the cpu lacks 3DNOW feature, we can use a normal prefetcht0 instruction instead of NOP5.
"prefetchw (%rxx)" and "prefetcht0 (%rxx)" have the same length, ranging from 3 to 5 bytes
depending on the register. So this patch even helps AMD64, shortening the length of the code.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>