cpu: Introduce env_archcpu

cpu: Introduce env_archcpu

This will replace foo_env_get_cpu with a generic definition.
No changes to the target specific code so far.

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