src/arch/x86/exit_car: Add proper x86_64 code

src/arch/x86/exit_car: Add proper x86_64 code

Don't truncate upper bits in assembly code and thus allow loading
of postcar stage above 4GiB.

Tested on qemu with cbmem_top set to TOUUD.

Change-Id: I42d1086f1220e44076ccf613244fc3c6d804805b
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Signed-off-by: Benjamin Doron <benjamin.doron@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79162
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>