x86: only allow real mode to access 32bit without LMA

x86: only allow real mode to access 32bit without LMA

When we're running in non-64bit mode with qemu-system-x86_64 we can
still end up with virtual addresses that are above the 32bit boundary
if a segment offset is set up.

GNU Hurd does exactly that. It sets the segment offset to 0x80000000 and
puts its EIP value to 0x8xxxxxxx to access low memory.

This doesn't hit us when we enable paging, as there we just mask away the
unused bits. But with real mode, we assume that vaddr == paddr which is
wrong in this case. Real hardware wraps the virtual address around at the
32bit boundary. So let's do the same.

This fixes booting GNU Hurd in qemu-system-x86_64 for me.

Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>