Also add custom load addresses for x86_64 target. This allows qemu to
load the kernel with -kernel argument. Without these addresses qemu
would refuse to load as it only supports 32 bit ELFs, but as our kernel
starts in 32 bit mode anyway, we can just load it!
This cleans up the kernel executable as bootloaders don't have to
load AP init code straight to 0xF000, but it will be moved there once
kernel is doing the AP initialization.
