tests: test more sizes for memcpy

Simple power of two is too easy.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
memcpy: aggressive unrolling

Copy < 256 bytes without any loops; 16 bytes and above use sse to reduce
instruction count.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>