XEN: EFI: Move x86 specific codes to architecture directory

Move x86 specific codes to architecture directory and export those EFI
runtime service functions. This will be useful for initializing runtime
service on ARM later.

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>