Fix s390x rootfs build

[janvorli]

Ubuntu for s390x architecture doesn't have OMP libraries. Remove them from rootfs build for s390x. Support for this architecture is in a very experimental stage and the OMP is needed for ML .NET only.

[janvorli]

The Linux leg failure is unrelated to this change. It seems that it is trying to use image that was recently removed. @mthalman is that one of those?

2021-07-12T11:34:45.8207074Z ##[command]/usr/bin/docker network prune --force --filter "label=b93516"
2021-07-12T11:34:45.9350721Z ##[command]/usr/bin/docker pull microsoft/dotnet-buildtools-prereqs:ubuntu-14.04-cross-0cd4667-20170319080304
2021-07-12T11:34:46.3766430Z Error response from daemon: pull access denied for microsoft/dotnet-buildtools-prereqs, repository does not exist or may require 'docker login': denied: requested access to the resource is denied

[janvorli]

Ah, I can see that a pending PR #7608 is fixing that issue.

[michaelgsharp]

LGTM.