build: export a meaningful arch-string to be used for OSv

Because we use "x64" while Linux will refer to "x86_64", I am drawing a
distinction between arch and arch_str - the later being the "official"
architecture string, from the uname PoV.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
use the canonical arch string for uname, instead of hard coding it

Fixes #395

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