Update PLATFORMS.md / re-enable CROSS on s390x

[SWilson4]

This PR ties up a couple of loose ends after #1960 and #1983:

• restore s390x and powerpc to PLATFORMS.md
• re-enable CROSS on s390x
• restore the Travis status badge

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged.)

[baentsch]

Thanks! Also resolves #1960 (comment)