build: allow side-by-side aarch64 and x64 builds

Build into build/release.x64 and build/release.aarch64 instead of
build/release.  A build/release symlink is retained to avoid confusion in
the transition period, as all the tooling plus everyone's fingers are used
to it.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
build: provide a default for CROSS_PREFIX

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
build: CROSS_PREFIX is always defined

Now that we provide a default, CROSS_PREFIX is always defined, so we can
drop the ifndef.

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