[tlbuild] Broken tests for mendexk on arm

[Karl Berry]

Johannes: For CI purposes, how about adding --disable-mendexk
--disable-upmendexk to the invocation on the failing platforms?

Since it may be some time until "someone" (anyone out there?) can debug,
or give me access to a machine where it fails (and I have time to look
at it).

Norbert, maybe you have such a machine in the Debian world and I can
foist this off onto you :)?

Thanks,
Karl