Update dotnet-build to produce portable layout

dotnet-build will produce a deps file for portable builds, and will now
create "runnable" outputs for RID-less targets

the outputs won't actually be runnable today because we need corehost
changes and to generate a deps.json file for corehost to use.

TestAssets/TestProjects/BuildTestStandaloneProject/Lib.cs

@@ -0,0 +1,3 @@
+public static class Thingy
+{
+}