[netcore] Marshal.IsPinnable (#13628)
Needed for MSBuild.
Stream changes were copied from [CoreRT](https://github.com/dotnet/corert/blob/master/src/System.Private.CoreLib/src/System/IO/Stream.CoreRT.cs).
IsPinnable - perhaps it should be an icall/intrinsics for better perf?
[CoreCLR impl for IsPinnable (InternalCall)](https://github.com/dotnet/coreclr/blob/
1f3f474a13bdde1c5fecdf8cd9ce525dbe5df000/src/vm/marshalnative.cpp#L257)
[CoreRT impl for IsPinnable](https://github.com/dotnet/corert/blob/master/src/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshal.cs#L114) (and [MightBeBlittable](https://github.com/dotnet/corert/blob/master/src/System.Private.CoreLib/src/System/Runtime/InteropServices/InteropExtensions.cs#L60))