Use current runtime + architecture for GenerateResource on .NET Core

[rainersigwald]

See dotnet/sdk#346 (comment).

On .NET Core, MSBuild doesn't support task hosts of different architectures/runtime versions. But the CoreResGen target does some gymnastics to figure out the right architecture/runtime version to use for the GenerateResource task.

On .NET Core we should just use CurrentArchitecture/CurrentRuntime.

Or as @nguerrera suggested, maybe just add the DoesTaskHostExist check for Architecture as well as Runtime.

[rainersigwald]

This is worked around for dev15. Pushing the better fix to later.

[dsplaisted]

Another wrinkle to this is that TargetFrameworkAsMSBuildRuntime is based on the TargetFrameworkVersion, and doesn't check the TargetFrameworkIdentifier. This means that for projects targeting .NET Core 2.0 or .NET Standard 2.0, MSBuild will try to generate resources using .NET Framework 3.5. If it's not installed, then it will fail with:

The "Microsoft.Build.Tasks.GenerateResource" task could not be loaded from the assembly Microsoft.Build.Tasks.v3.5, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a. Could not load file or assembly 'Microsoft.Build.Tasks.v3.5, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified. Confirm that the declaration is correct, that the assembly and all its dependencies are available, and that the task contains a public class that implements Microsoft.Build.Framework.ITask.