tests: cpp: libcxx: Test with full and nano variants of newlib

In order to increase test coverage, this commit updates the libcxx test
to run with both full and nano variants of the newlib.

Note that C++ exception handling feature is only enabled for the newlib
full variant because the nano variant does not support C++ exception
handling.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>