Wrap mono_gc_init_finalizer_thread() call with now-needed coop GC sta…

…te transition. (xamarin#8248)

Starting with mono/mono#16907 , the Mono runtime ends in GC-Safe state (mode) after mono_jit_init_version() is called. The monotouch code did not expect that, and calls mono_gc_init_finalizer_thread() which also tries to enter GC-safe mode.

Should fix mono/mono#19372

Co-authored-by: Alexis Christoforides <alexis@thenull.net>

runtime/monotouch-main.m

@@ -203,7 +203,9 @@ - (void) start
 {
 	// COOP: ?
 #if defined (__arm__) || defined(__aarch64__)
+	MONO_ENTER_GC_UNSAFE;
 	mono_gc_init_finalizer_thread ();
+	MONO_EXIT_GC_UNSAFE;
 #endif
 }