Draft: community/g++-cross-embedded: add xtensa support
This adds g++ support for popular xtensa based MCUs. It makes use of xtensa-dynconfig to avoid having duplicated host side tools, but the libstdc++ is installed for each target separately.
Follow-up to: !66926 (merged)
Note: This is currently on halt until the !66926 (merged) has made its way through the builders and reached the repos, as otherwise the CI will fail.
Also note: Merging this will keep the builders busy for quite a while. It may make sense to schedule the merge when there is not too much backlog.