x86: Add Kconfig options to build 64-bit U-Boot

Add a new CONFIG_X86_64 option which will eventually cause U-Boot to be
built as a 64-bit application, with SPL doing the 16/32-bit init.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>