Crossgen Shared Framework managed assemblies

- Use Crossgen's Ready To Run mode on all of the managed assemblies
  that make up the shared framework.
- Upgrade the version of the shared framework to match what is used
  in the rest of CLI (see the comment in SharedFrameworkTargets.cs to
  understand why this is needed).
- Remove the IL mscorlib.dll image, since the Crossgen'd image is
  already published.

Fixes dotnet/corefx#6753

scripts/dotnet-cli-build/Utils/Crossgen.cs

@@ -0,0 +1,50 @@
+using System.IO;
+using System.Linq;
+using System;
+using System.Runtime.InteropServices;
+using Microsoft.DotNet.Cli.Build.Framework;
+using Microsoft.Extensions.PlatformAbstractions;
+
+namespace Microsoft.DotNet.Cli.Build
+{
+    internal static class Crossgen
+    {
+        public static string GetCrossgenPathForVersion(string coreClrVersion)
+        {
+            string arch = PlatformServices.Default.Runtime.RuntimeArchitecture;
+            string packageId;
+            if (CurrentPlatform.IsWindows)
+            {
+                packageId = $"runtime.win7-{arch}.Microsoft.NETCore.Runtime.CoreCLR";
+            }
+            else if (CurrentPlatform.IsUbuntu)
+            {
+                packageId = "runtime.ubuntu.14.04-x64.Microsoft.NETCore.Runtime.CoreCLR";
+            }
+            else if (CurrentPlatform.IsCentOS || CurrentPlatform.IsRHEL)
+            {
+                // CentOS runtime is in the runtime.rhel.7-x64... package.
+                packageId = "runtime.rhel.7-x64.Microsoft.NETCore.Runtime.CoreCLR";
+            }
+            else if (CurrentPlatform.IsOSX)
+            {
+                packageId = "runtime.osx.10.10-x64.Microsoft.NETCore.Runtime.CoreCLR";
+            }
+            else if (CurrentPlatform.IsDebian)
+            {
+                packageId = "runtime.debian.8.2-x64.Microsoft.NETCore.Runtime.CoreCLR";
+            }
+            else
+            {
+                return null;
+            }
+
+            return Path.Combine(
+                Dirs.NuGetPackages,
+                packageId,
+                coreClrVersion,
+                "tools",
+                $"crossgen{Constants.ExeSuffix}");
+        }
+    }
+}