Fix nuget gen on multi-core machines, fix build issue when not building for coreclr

Fix build issue caused by eliminating coreclr from default build.  Add an environment variable to add it bck

skip more coreclr steps on plain build.cmd

Don't build coreclr by default for plain ``build.cmd``

Update dev15-sxs branch with latest from master

Merge Coreclr support into FSharp Mainline

This reverts commit cb453cc1.

More test fixes for coreclr branch

Fix delay signing for FSharp.Compiler.Unittests assembly

Fixes: 1. nuget build to support msbuild with /m. 2. Some FSharpqa tests that fail in the absense of .Net 2.0

show that #1020 is fixed

Conflicts:
	tests/fsharpqa/Source/InteractiveSession/Misc/env.lst