[libaxx] Explicitely define __cxa_pure_virtual

Change-Id: I07315f1bf84771a2784788e0d0eea4360245701d

libaxx/Makefile

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

libaxx/src/cxxabi/pure_virtual.cpp

@@ -0,0 +1,10 @@
+extern "C" {
+#include <stdlib.h>
+}
+
+/* C++ code calls __cxa_pure_virtual when a pure-virtual method is called.
+ * This is an error case, so we just redirect it to abort. */
+
+extern "C" void __cxa_pure_virtual() {
+  abort();
+}