Use the crossgen and jit that comes with Microsoft.NETCore.App so we don't have to keep a corresponding CoreCLR version around.
Remove workarounds for crossgen bug https://github.com/dotnet/coreclr/issues/9118
* Eliminate CleanPublishOutput
* Decompose Crossgen Task
* WiP
* TarGzFileExtractToDirectory
* FixModeFlags --> CHMod
Also various eliminations of dead code
* Tasks cleanup
Move all tasks to .tasks file. There is little value in keepint them in each source file as they are already being used assumptively by files that happen to get executed later.
Also eliminating uses of <Exec> for DotNet invocations
* Move to BuildTools implementation of TarGzCreateFromDirectory
* Eliminate Command.cs and helpers
* Remove dead code
* Revert TarGz from BuildTools
Latest build tools package has not picked up the task, though it is checked in.
* Disable ChMod on Windows
* Windows bug fix
* PR Feedback
* Finish changing Chmod caps
