x86: Use the existing GDT in the ROM for 64-bit U-Boot proper

It is unnecessary to use a RAM version GDT for 64-bit U-Boot proper.
In fact we can just use the ROM version directly, which not only
eliminates the risk of being overwritten by application, but also
removes the complexity of patching the cpu_call64().

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