v1.21.14-rc1+k3s1

Fix download script; add arm/arm64 (#7)

* Download should fail if curl fails
* Add arm/arm64 arch
  rancher/system-agent supports these platforms, as does k3s. This repo should as well.
* Handle s390x on old branches and bump fallback version
* Simplify bin suffix logic and handle arm properly

Signed-off-by: Brad Davidson <brad.davidson@rancher.com>