Add building luac.cross to travis ci builds

[HHHartmann]

Give it a spin on all lua files afterwards

Fixes #2681.

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[HHHartmann]

I thought about that too but did not want to generate a bunch of extra scripts.
Also the common part would be ugly to implement depending on a variable being set in the platform dependent part.
Also more testing would have to be done as I am not so sure how to make the script break on any error.

If going for scripts I would like to implement a common naming convention or even an extra directory.
tools/travis maybe?

I probably will not find time to work on this in the next couple of weeks so maybe adding scripts could be deferred to a followup PR.

[HHHartmann]

How can I trigger a travis build for this

[HHHartmann]

Travis-CI build succeeds, so could this be accepted?