[tlbuild] expand_depth vs. sigsegv check

[Karl Berry]

    aarch64-linux, 512 MB RAM:
    This is TeX, Version 3.14159265 (TeX Live 2020) (preloaded format=tex)
    ! TeX capacity exceeded, sorry [expansion depth=10000].

Thanks. A "small" machine like that was what I was most curious
about. The fact that 10000 is small enough even for that system is good
news.

Ken B has already rebuilt the Cygwin binaries with a larger stack size,
so that expand_depth=10000 should now result in the nice TeX error there
as well. Unless I hear otherwise, I'm figuring those were the only
systems where the problem was happening.

Thanks to all for your responses. -k