We can certainly support some common tests for system emulation that
make use of our minimal defined boot.S support. It will still be up to
individual architectures to ensure they build so we provide a
MULTIARCH_TESTS variable that they can tack onto TESTS themselves.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>