Generate packages for more compiler versions on Linux

[kyllingstad]

Currently, we only do CI builds using GCC 9 on Linux. To make life easier for our users, we should expand the compiler selection and host a greater variety of packages on osp.jfrog.io.

If #747 gets merged, this will become way easier. It switches the Docker image used for build containers to conanio/<compiler><version>-ubuntu16-04, so it's just a matter of extending our build matrix.

I would suggest we build packages using both GCC and Clang and, say, the five most recent major versions of each compiler. We don't need to do this for every PR, just for the main branch. PR builds can be restricted to the oldest of the compiler versions to save time.