[runtimes] Fix umbrella component targets

When we're building the runtimes for multiple platform targets, we
create umbrella build targets for each distribution component, but those
targets didn't have any dependencies and were just no-ops. Make the
umbrella target depend on the sub-targets for each platform to fix this,
which is consistent with the behavior of the umbrella targets for each
runtime, and also consistent with the behavior when we've only specified
the default target.