We will soon need to use common functionality(*) from both runtimes and
other packages. The "other packages" are all C++ and it is handy to keep
common functionality in C++ as well. While we could also maintain
`extern "C"` interface, it duplicates the work. Let's switch everything
to C++ to ease further maintenance.

(*) e.g. package internal/syscall from both runtimes and from package os.

/reviewed-on !17