[tlbuild] Broken tests for mendexk on arm

[Karl Berry]

    test if my upmendex binary

Ah. I fixed mendex (yes, the same test failed for me and yes, it
succeeded after my change), but not upmendex. I forgot there are two
programs. (Aside: The massive code duplication throughout TeX and TL is
a maintenance nightmare.)

It will take a long time for Nelson's system to compile icu and
upmendex.  I started it running. In the meantime, I suggest just making
the same change (char -> int) in upmendex/mendex.h and ... fingers crossed.

Thanks,
Karl