arm: add __aeabi_atexit()

arm eabi requires this symbol for static C++ dtors.
usually it is provided by libstdc++, but when a C++ program
doesn't use the std lib (free-standing), the libc has to provide
it.
this was encountered while building transmission, which
depends on such a C++ library (libutp).

this function is nearly identical to __cxa_atexit, but it has the
order of argumens swapped for "performance reasons".
see page 25 of

 http://infocenter.arm.com/help/topic/com.arm.doc.ihi0043d/IHI0043D_rtabi.pdf

there are other aeabi specific C++ support functions missing, but
it is not clear yet that GCC makes use of them so we omit them for
the moment.

arch/arm/src/__aeabi_atexit.c

@@ -0,0 +1,6 @@
+int __cxa_atexit(void (*func)(void *), void *arg, void *dso);
+
+int __aeabi_atexit (void *obj, void (*func) (void *), void *d)
+{
+	return __cxa_atexit (func, obj, d);
+}