objtool: Add xen_start_kernel() to noreturn list

xen_start_kernel() doesn't return.  Annotate it as such so objtool can
follow the code flow.
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/930deafa89256c60b180442df59a1bbae48f30ab.1611263462.git.jpoimboe@redhat.com

tools/objtool/check.c

@@ -160,6 +160,7 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
 		"machine_real_restart",
 		"rewind_stack_do_exit",
 		"kunit_try_catch_throw",
+		"xen_start_kernel",
 	};
 
 	if (!func)