arch/arc: Remove redundant __cxa_atexit from MWDT stubs

__cxa_atexit was originally defined by the IA64 C++ ABI, but it has been
adopted by other C++ implementations since then.  This function is already
present in lib/cpp/api/cpp_dtors.c, which is built by default when C++
support is enabled, so we don't need another copy in the MWDT-specific
code.

Signed-off-by: Keith Packard <keithp@keithp.com>