Improve default RID selection for .NETFramework exes

[nguerrera]

We currently pick win7-x86 or win7-x64 (depending on PlatformTarget) when building on Windows. We should make this more dynamic based on host OS, particularly when we support building net4x applications for non-Windows using Mono.

There are too many uses of $(RuntimeIdentifier) and $(RuntimeIdentifers) in static evaluation context for us to simply run some code to pick a RID.

Possible fixes:

1. msbuild sets the host rid
2. nuget uses/returns host rid as part of restore data
3. things are refactored such that the rid is not needed statically

[livarcocc]

@dsplaisted do we need this for the nuget reference assemblies work you are doing? Or is it ok to move this issue?

[dsplaisted]

@livarcocc This shouldn't be directly related to making reference assemblies available as NuGet packages. You can move it out.