x86: Move work-around out of cpu_jump_to_64bit_uboot()

At present this function copies U-Boot from the last 1MB of ROM. This is
not the right way to do it. Instead, the binman symbol should provide the
location.

But in any case the code should live in the caller,
spl_board_load_image(), so that the 64-bit jump function can be used
elsewhere. Move it.

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