Add --ilcsdkpath support for native compilation

[gkhanna79]

This will enable to consume a custom build of ILCompiler.SDK when compiling an app.

Associated with PR: dotnet/cli#622

[schellap]

We are already using the recently built ILC SDK in our build system in the project.json and issuing a publish on the just built SDK. Why is this an issue in the first place and why is this change necessary?

[schellap]

If you want to have a cross compiler, this is not the right fix. We should relayout the package such that when we issue publish (with @brthor 's support to publish) we end up with the following structure:

x64<ILC SDK contents>
x64_arm64<ILC SDK contents>

[gkhanna79]

Fixed (see PR referenced above).