S390x support

[bhess]

• Adds build support for s390x
• Simple CI integration with TravisCI
• Small fix in Keccak code (unused variable on big endian architectures)
• Small fix in FrodoKEM-SHAKE code to support big endian (see Endianness issues with BIKE #1101)
• TravisCI build enables all algorithms except BIKE which doesn't support big endian (see Endianness issues with BIKE #1101)

Note: the generic build sets -march=z10 as default (latest architecture is z15). This is also the default and minimum architecture supported by clang. This minimum requirement ensures that there is a full-resolution clock (stckf) available for benchmarking.

Fixes #1100

• 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 the list of algorithms available -- either adding, removing, or renaming? (If so, PRs in OQS-OpenSSL, OQS-BoringSSL, and OQS-OpenSSH will also be required by the time this is merged.)

[baentsch]

Thanks for making Frodo run on the mainframe. That makes BIKE "the odd man out" and forces me to ask the question whether it'd be sensible to change our documentation as part of this PR: "Supported architectures: All" now doesn't sound right any more for BIKE.

[bhess]

@baentsch Thank you for the follow-up in #1104.