Integration of minor fixes from the build-infra project

The build-infra project has collected a number of minor fixes and changes during the new hotspot build development. It's a mix of code cleanup and new capabilities.

Not all of these new features are immediately beneficial to the JDK, but they will be needed for the upcoming new Hotspot build, and it will not hurt to have them in mainline. (In fact, it will tremendously help merging between mainline and build-infra.)