arm: selftest: rename svc mode to kernel mode

Separate the concepts of an 'svc', the syscall instruction present
on both arm and arm64, and 'svc mode', which is arm's kernel mode,
and doesn't exist on arm64. kernel mode on arm64 is modeled with
exception level 1 (el1).

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>