efi: efivars: limit availability to X86 builds

CONFIG_EFI_VARS controls the code that exposes EFI variables via
sysfs entries, which was deprecated before support for non-Intel
architectures was added to EFI. So let's limit its availability
to Intel architectures for the time being, and hopefully remove
it entirely in the not too distant future.

While at it, let's remove the module alias so that the module is
no longer loaded automatically.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>