uncaught exceptions is not yet implemented

[wolfmanjm]

This is called in ostreams...
https://github.com/Smoothieware/smoothie-nuttx/blob/master/nuttx/include/libcxx/ostream#L279
and causes an abort.

Temporary workaround is to return 0 in uncaught_exceptions() in libcxx/exception.cxx when nuttx is defined