x86/realmode: build with -D__DISABLE_EXPORTS

Commit 156ff4a5 ("x86/ibt: Base IBT bits") added this option when
building realmode in order to disable IBT there. This is also needed in
order to disable return thunks.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>