[dotnet-linker] Add TargetFramework to the configuration and set Driver.TargetFramework.

This way we can use the shared Application.Platform implementation.

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -137,6 +137,7 @@
 				PlatformAssembly=$(_PlatformAssemblyName).dll
 				SdkVersion=$(_SdkVersion)
 				TargetArchitectures=$(TargetArchitectures)
+				TargetFramework=$(_ComputedTargetFrameworkMoniker)
 				Verbosity=$(_BundlerVerbosity)
 			</_CustomLinkerOptions>
 			<_ExtraTrimmerArgs>$(_ExtraTrimmerArgs) --custom-data "LinkerOptionsFile=$(_CustomLinkerOptionsFile)"</_ExtraTrimmerArgs>

tools/dotnet-linker/Compat.cs

@@ -37,10 +37,6 @@ namespace Xamarin.Bundler {
 		{
 			throw new NotImplementedException ();
 		}
-
-		public ApplePlatform Platform {
-			get { return Configuration.Platform; }
-		}
 	}
 
 	public partial class Driver {

tools/dotnet-linker/LinkerConfiguration.cs

@@ -120,6 +120,11 @@ namespace Xamarin.Linker {
 							Abis.Add (a);
 					}
 					break;
+				case "TargetFramework":
+					if (!TargetFramework.TryParse (value, out var tf))
+						throw new InvalidOperationException ($"Invalid TargetFramework '{value}' in {linker_file}");
+					Driver.TargetFramework = TargetFramework.Parse (value);
+					break;
 				case "Verbosity":
 					if (!int.TryParse (value, out var verbosity))
 						throw new InvalidOperationException ($"Invalid Verbosity '{value}' in {linker_file}");
@@ -149,6 +154,9 @@ namespace Xamarin.Linker {
 			default:
 				break;
 			}
+
+			if (Driver.TargetFramework.Platform != Platform)
+				throw ErrorHelper.CreateError (99, "Inconsistent platforms. TargetFramework={0}, Platform={1}", Driver.TargetFramework.Platform, Platform);
 		}
 
 		public void Write ()