[mtouch] Clear the MONO_THREADS_SUSPEND environment variable before calling the AOT compiler. Works around mono/mono#11765.

Visual Studio for Mac may set MONO_THREADS_SUSPEND, and this ends up confusing
the AOT compiler when compiling for watchOS.

So unset the environment variable before calling the AOT compiler.

Works around https://github.com/mono/mono/issues/11765.

tools/mtouch/Assembly.cs

@@ -290,6 +290,12 @@ namespace Xamarin.Bundler {
 				ProcessStartInfo = Driver.CreateStartInfo (App, aotCompiler, aotArgs, Path.GetDirectoryName (assembly_path)),
 				AotInfo = aotInfo,
 			};
+			if (App.Platform == ApplePlatform.WatchOS) {
+				// Visual Studio for Mac sets this environment variable, and it confuses the AOT compiler.
+				// So unset it.
+				// See https://github.com/mono/mono/issues/11765
+				task.ProcessStartInfo.EnvironmentVariables ["MONO_THREADS_SUSPEND"] = null;
+			}
 
 			aotInfo.Task = task;
 		}