Update dist-various to Ubuntu 20.04

This updates the dist-various-1 and dist-various-2 images to Ubuntu
20.04. This requires some adjustments:

 * `DEBIAN_FRONTEND=noninteractive` required for apt install.
 * `team-gcc-argm-embedded` PPA does not support focal. However,
   we can simply use the distro-provided `gcc-arm-none-eabi`. Per
   the comment, the PPA was only used to get a newer version.
 * rumprun has to be updated to avoid a linker error.
 * We need to build rumrun with `NOGCCERROR`, which disables use
   of `-Werror` and allows building with a newer compiler.
 * We need to install `libtinfo5`, which appears to be a dependency
   of the clang used during the fuchsia build.
 * We need to switch to `g++-8` rather than `g++-7`, as at least
   `g++-7-arm-linux-gnueabi` is not available on focal.
 * We need to upgrade to GCC 6.5 for the Solaris build, as GCC 6.4
   does not support the newer libisl version.

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