* Update .NET SDK to 2.1.300-rtm-008866

* Set TargetLatestRuntimePatch to false for ILAsm

* Kill build processes after boostrapping phase

* Fix SDK tests

This moves Roslyn to use the 2.1 SDK.

This may appear to be moving the SDK version backwards from 2.2 to 2.1.
In reality though it is a move forward. There was a brief time when the
SDK switched their versions to 2.2 before reverting back to 2.1. Roslyn
took a dependency on SDK in that small window.

More details available here:

https://github.com/dotnet/announcements/issues/57