Enable ildasm for *nix

Enable ildasm for *nix

This enables ildasm for cross-platforms.
Unlike Window (where initialization is done when DLL attach), CoreCLR is explciltly hosted/initialized to get the Metadata related APIs.
This also eliminates the need of setting dynamic library path.
Currently, ildasm binary is assumed to be located same as CoreCLR binary.
I added a simple CoreCLRLoader (not meant to run an assembly file) for just direct uses of APIs. Since I expect this library to be used for ilasm work.
Resource string is also handled using a static string table  based on my prior check-in.
Other changes are mostly mechanic with regard to wide constant string.