run_tests/mkstandalone: add arch_cmd hook

This allows us, for example, to auto generate a new test case based on
an existing test case.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Message-Id: <20200923134758.19354-4-mhartmay@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>