Stub cibuild.cmd to enable multi-runtime CI builds
Stub cibuild.cmd to enable multi-runtime CI builds

Checking this into xplat so that cf10c8a9 has a cibuild.cmd script to
target in the xplat branch. As-is, it always builds for .NET Core, even if
asked to build for desktop. A follow-up, full-featured cibuild.cmd will be
capable of building both desktop and Core.

mono/msbuild,rainersigwald/msbuild,chipitsine/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,Microsoft/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,Microsoft/msbuild,chipitsine/msbuild,chipitsine/msbuild,mono/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild,cdmihai/msbuild,mono/msbuild,cdmihai/msbuild,jeffkl/msbuild,Microsoft/msbuild,jeffkl/msbuild,chipitsine/msbuild,mono/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,Microsoft/msbuild,rainersigwald/msbuild