add forward declaration to __cxa_atexit to aeabi_atexit, to prevent warnings

newlib/libc/sys/arm/aeabi_atexit.c

@@ -1,5 +1,8 @@
 #include <stdlib.h>
 
+/* forward declaration */
+extern int __cxa_atexit (void (*) (void *), void *, void *);
+
 /* Register a function to be called by exit or when a shared library
    is unloaded.  This routine is like __cxa_atexit, but uses the
    calling sequence required by the ARM EABI.  */