cpu: Define ArchCPU

cpu: Define ArchCPU

For all targets, do this just before including exec/cpu-all.h.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>