[Elftoolchain-developers] Ubuntu 17.10 is EOL

[Ed M. <em...@fr...>]

It seems that Ubuntu 17.10 is our currently supported Linux version,
but it went EOL as of July 2018. 18.04 is likely a good choice as it
is supported until 2028. However, I don't normally use Linux; it would
be good if someone can make sure ELF Tool Chain builds on 18.04 and
then add a build config to CI (.cirrus.yml).