mkpost.go replaces all variables prefixed with 'X_' with '_' on s390x
because most of them do not need to be exposed. X__val is being used
by a third party library so it turns out we do need to expose it on
s390x (it is already exposed on all other Linux architectures).
Fixes#17298 and updates #18632.
Reviewed-by: Brad Fitzpatrick <email@example.com>