arch/x86/lib: Fix CONFIG_X86_64 usage in zimage.c

arch/x86/lib: Fix CONFIG_X86_64 usage in zimage.c

Correct the preprocessor directive used to check for 64-bit kernel
support in the `zboot_go` function. The code previously checked for
`CONFIG_X86_RUN_64BIT`, which is not the correct configuration option
for determining if the kernel should run in 64-bit mode. The correct
option is `CONFIG_X86_64`.

Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>