.NET Core libraries should reference System.Runtime instead of mscorlib

[Osman Turan]

Dynamically generated assemblies reference System.Private.CoreLib assembly first. System.Private.CoreLib public key token is same as mscorlib. Since, .NET Core is fundamentally different from .NET Framework (.NET Core supports side-by-side runtime), we shouldn't reference mscorlib first. Instead, we should reference System.Runtime assembly which we should extract at its metadata at runtime. Also, we should map all mscorlib and System.Private.CoreLib references to System.Runtime.