Fix PostgreSQL and MySQL setup on Travis CI

Note that the PostgreSQL tests still don't work on some architectures.

Closes GH-5607.

Add S390X architecture as a Travis job

This gives us a way to compile and test a big endian architecture.

Closes GH-5382.

Add ARM64 CI to Travis

We need to install a number of additional packages that are installed
by default on the AMD64 workers.

We also have to manually set up the MySQL user.

Add ARM64 CI to Travis

We need to install a number of additional packages that are installed
by default on the AMD64 workers.

We also have to manually set up the MySQL user.

For now we don't set up Postgres -- if anyone wants to figure that
out, it would be great ;)

Log redirections in compile.sh are removed, because /dev/stdout is
not accessible. We don't see to use this anyway.

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Simplify travis setup scripts

Reduce duplication.