x86/boot: Fix data placement around __high_start()

x86/boot: Fix data placement around __high_start()

multiboot_ptr should be in __initdata - it is only used on the BSP path.
Furthermore, the .align 8 then .long means that stack_start is misaligned.

Move both into setup.c, which lets the compiler handle the details correctly,
as well as providing proper debug information for them.

Declare stack_start in setup.h and avoid extern-ing it locally in smpboot.c.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>