x86_64: Always use builtin memcpy on gcc 4.3

Jan asked to always use the builtin memcpy on gcc 4.3 mainline because
it should generate better code than the old macro. Let's try it.

Cc: Jan Hubicka <jh@suse.cz>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>