Add intrinsic cxx attribute as preferred alternative to cpp

[mojavelinux]

CXX, rather than CPP, is the more common alpha-only shorthand for C++. There are two reasons. First, xx can be read as two rotated pluses. Second, pp is already reserved in that ecosystem to mean "preprocessor".

The intrinsic attribute in AsciiDoc should honor existing terminology by offering cxx as the preferred alternative to cpp.

I don't think we need to add xx in place of pp as the later is generally accepted as an alpha-only shorthand for ++ for other languages such as SQL++. However, someone is welcome to come forward to make the case otherwise.