[linker] Don't remove RuntimeCompatibilityAttribute from assembly

It's needed at runtime since it changes exception handling behavior.

The "link sdk" Linker_RuntimeWrappedException() test relies on it,
but this never showed up there since "link sdk" doesn't link the main assembly.

tests/linker/ios/link sdk/LinkSdkRegressionTest.cs

@@ -386,7 +386,7 @@ namespace LinkSdk {
 		}
 		
 		[Test]
-		// http://lists.ximian.com/pipermail/monotouch/2011-December/006976.html
+		// http://lists.dot.net/pipermail/monotouch/2011-December/006976.html
 		public void Linker_RuntimeWrappedException ()
 		{
 			try {

tools/linker/MonoTouch.Tuner/RemoveAttributes.cs

@@ -71,7 +71,7 @@ namespace MonoTouch.Tuner {
 			"System.Runtime.CompilerServices.DecimalConstantAttribute",
 			"System.Runtime.CompilerServices.DefaultDependencyAttribute",
 			//"System.Runtime.CompilerServices.ExtensionAttribute", used at runtime by LINQ, see bug #3028
-			"System.Runtime.CompilerServices.RuntimeCompatibilityAttribute",
+			//"System.Runtime.CompilerServices.RuntimeCompatibilityAttribute", used at runtime by runtime wrapped exception handling
 			"System.Runtime.CompilerServices.StringFreezingAttribute",
 			"System.Runtime.ConstrainedExecution.PrePrepareMethodAttribute",
 			"System.Runtime.ConstrainedExecution.ReliabilityContractAttribute",