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.

Change-Id: Ic03463229a5f75ca41a4a4b50300da4b4d892d45
Reviewed-on: https://go-review.googlesource.com/30130Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>