Treat failure to resolve a toolchain as if it were an incompatible target for skipping in ... expansion.

Incompatible target skipping will skip targets that can not be built for the specified destination platform.
There is also a need to be able to skip targets which can not be built on the execution platform. For example, because a test might depend on an OS native tool which makes no sense on any other platform, or a compiler is only licensed to a few users in an organization.

It would be convenient if failure to resolve a toolchain (usually because one was not registered) as a skippable target.

TBD: Bring in more context from the mail thread which sprouted this idea.

cc: @gregestren @AustinSchuh @philsc @katre

The issue names incompatible target skipping and toolchain resolution as the starting points; first recover the mail-thread context it references. Done means an agreed behavior and an implementation and test location for treating unresolved toolchains as skippable, including execution-platform-only dependencies.

Written by the indexing model from the issue text.