arch_init.h: Don't include arch_init.h unnecessarily

arch_init.h only defines the QEMU_ARCH_* enumeration and the
arch_type global. Don't include it in files that don't use those.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210730105947.28215-8-peter.maydell@linaro.org