x86/realmode: build with -D__DISABLE_EXPORTS

stable inclusion
from stable-v5.10.133
commit accb8cfd506da1e218a0cd56a11f37885dc32adf
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5PTAS
CVE: CVE-2022-29900,CVE-2022-23816,CVE-2022-29901

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=accb8cfd506da1e218a0cd56a11f37885dc32adf



--------------------------------

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>
Signed-off-by: Lin Yujun <linyujun809@huawei.com>
Reviewed-by: Zhang Jianhua <chris.zjh@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>