Proposal: Use C++ instead of C

[gacholio]

I propose we stop using .c files in OpenJ9 in favour of .cpp files. I am not proposing an actual language shift - most files would be wrapped in extern "C" rather than making them into classes. The advantages are:

• inline variable declarations on every platform
• inline functions are much more flexible and readable than preprocessor macros

Conversion can take place one file at a time. The majority of the work will be adding casts because C++ does not allow implicit conversion to/from void*.

Any new files should be .cpp.