README: build requires 64-bit distribution

OSv can only be built on a 64-bit Linux distribution - otherwise the
compiler will not (by default, at least) generate 64-bit code, and we
won't be able to run the 64-guest needed for building the ZFS filesystem.

Make this requirement explicit in README.md.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>