asm: Move builtin_memcpy helper into asm/string.h

There will be a couple of more builtin helpers needed
in pie code soon. Thus to unify approach do

 - rename asm/memcpy_64.h to asm/string.h

 - introduce include/asm-generic/string.h file
   where all helpers are implemented if optimized
   variant is not  yet provided

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>