[libaxx] Define __cxa_atexit

In practice, it does nothing!

Change-Id: I5fb92299c4672d1635d0b77c2fc5de4e666c7213

libaxx/Makefile

@@ -1,2 +1,2 @@
 objs += $(addprefix libaxx/src/, new.o)
-objs += $(addprefix libaxx/src/cxxabi/, pure_virtual.o)
+objs += $(addprefix libaxx/src/cxxabi/, atexit.o pure_virtual.o)

libaxx/src/cxxabi/atexit.cpp

@@ -0,0 +1,14 @@
+/* C++ expects the __dso_handle symbol to be defined to some unique value in
+ * each dynamic shared object. Even though we're not using dynamic loading,
+ * we still have to define __dso_handle. */
+
+void * __dso_handle = nullptr;
+
+/* The __cxa_atexit function registers a function to be called when the program
+ * exits or when a shared library is unloaded.
+ * We don't support shared libraries and our program should never exit, so we
+ * can simply do nothing and return zero. */
+
+extern "C" int __cxa_atexit(void (*dtor)(void *), void * object, void * handle) {
+  return 0;
+}