x64: Switch to "-msse2" GCC option

VirtualBox, for example, does not enable SSE 4.1 by default and Avi
tells me QEMU does not do that either.

To make OSv always bootable, switch to SSE2 which is guaranteed to be
present on x86-64.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>