Fix where the real mode interrupt vector ends

The real mode ends at 0x400, not 0x100. The code intentionally excludes
the IVT as RAM, so use the correct address.

Also, 0x100 is not 1K aligned and will be rejected by add_memmap(). We
have observed problems that after a multiboot2 kexec, the next kexec
will throw away such unaligned chunks, losing memory for the next next
kernel. In some corner cases, such loss of memory can actually cause OOM
during boot.

Signed-off-by: Hongyan Xia <hongyxia@amazon.com>
Signed-off-by: Simon Horman <horms@verge.net.au>