Add cpu_relax() for s390 and x86 for use in arch-agnostic tests.  arm64
already defines its own version.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220226001546.360188-28-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>