[runtime] Update thread attach/detach according to mono changes.

* Restore the mono_jit_thread_attach signature to how it's in
  the latest stable.

* Don't use the new attach/detach methods when using the dynamic
  mono runtime, since the methods aren't available in any released
  version of mono. Also XM doesn't support coop yet, so it's not
  needed.

* And finally don't use the new attach/detach methods for anything
  that isn't building using mono/master (iow only watchOS). This
  should be changed once the rest of the products starts using
  mono/master again.

runtime/exports.t4

@@ -516,13 +516,7 @@
 		#region mini/mini.h
 
 		new Export ("void*", "mono_jit_thread_attach",
-			"MonoDomain *", "domain",
-			"void *", "dummy"
-		),
-
-		new Export ("void", "mono_jit_thread_detach",
-			"void *", "cookie",
-			"void *", "dummy"
+			"MonoDomain *", "domain"
 		),
 
 		new Export ("gboolean", "mono_exception_walk_trace",
@@ -601,6 +595,20 @@
 		#region utils/mono-threads.h
 		new Export ("void*", "mono_thread_info_current_unchecked" ),
 		#endregion
+
+		#region metadata/threads-types.h
+
+		new Export (true, "void *", "mono_threads_attach_coop",
+			"MonoDomain *", "domain",
+			"gpointer*", "dummy"
+		),
+
+		new Export (true, "void *", "mono_threads_detach_coop",
+			"gpointer", "cookie",
+			"gpointer*", "dummy"
+		),
+
+		#endregion
 	};
 #><#+
 	class Arg

runtime/xamarin/runtime.h

@@ -258,15 +258,26 @@ class XamarinObject {
 
 #endif /* !TARGET_OS_WATCH */
 
+// Once we have one mono clone again the TARGET_OS_WATCH
+// should be removed (DYNAMIC_MONO_RUNTIME should still
+// be here though).
+#if TARGET_OS_WATCH || DYNAMIC_MONO_RUNTIME
 #define MONO_THREAD_ATTACH \
 	do { \
 		gpointer __thread_dummy; \
-		gpointer __thread_cookie = mono_jit_thread_attach (NULL, &__thread_dummy) \
+		gpointer __thread_cookie = mono_threads_attach_coop (NULL, &__thread_dummy) \
 
 #define MONO_THREAD_DETACH \
-		mono_jit_thread_detach (__thread_cookie, &__thread_dummy); \
+		mono_threads_detach_coop (__thread_cookie, &__thread_dummy); \
 	} while (0)
+#else
+#define MONO_THREAD_ATTACH \
+	do { \
+		mono_jit_thread_attach (NULL) \
 
+#define MONO_THREAD_DETACH \
+	} while (0)
+#endif
 
 #ifdef __cplusplus
 } /* extern "C" */