add startup abi functions, dummy for now. eventually needed for c++ support.

src/env/__libc_csu_fini.c

+#include "libc.h"
+
+void __libc_csu_fini()
+{
+}

src/env/__libc_csu_init.c

+#include "libc.h"
+
+void __libc_csu_init(int argc, char **argv, char **envp)
+{
+}