xap_helper_cxx: -O2 optimize read-only files by default

While fast build times from -O0 is critical to my sanity when
actively working on C++, the files installed via package
managers or `make install' aren't likely to change frequently.

In that case, expensive -O2 optimizations make sense since the
10-20s saved from a single large --join more than covers the
cost of waiting on g++ to optimize.

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