efi: Rename armxx to arch

efi: Rename armxx to arch

Some architectures want to boot Linux as plain UEFI binary. Today that
really only encompasses ARM and AArch64, but going forward more
architectures may adopt that model.

So rename our internal API accordingly.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>