Kernel: Cleanup bootloader headers

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!

kernel/arch/i686/linker.ld

@@ -11,6 +11,7 @@ SECTIONS
 	{
 		g_kernel_execute_start = .;
 		*(.multiboot)
+		*(.multiboot2)
 		*(.bananboot)
 		*(.text.*)
 	}