Enable `nvq++` to recognize C++ source files with `.cxx` extension.

[boschmitt]

Required prerequisites

• [X] Search the issue tracker to check if your feature has already been mentioned or rejected in other issues.

Describe the feature

The C++ standard does not mandate any extension for C++ source files and thus tools typically recognize a set of them. nvq++ can recognize both .cpp and .cc, however it does not recognize .cxx.

The .cxx extension plays an important role in CMake: when testing a C++ compiler capabilities, CMake will try to compile a bunch of test files that have this extension. Hence, for downstream CMake integration, it will be important to correctly handle source files with this extension.