Like FEAT_TRF (Self-hosted Trace Extension), suppress tracing
external to the cpu, which is out of scope for QEMU.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20230811214031.171020-10-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>