Enable PIC even for static libraries
    
    Start using CMAKE_POSITION_INDEPENDENT_CODE=ON in order to keep the same
    behavior that existed before a set of changes modified it and ended up
    with the inclusion of CMAKE_POSITION_INDEPENDENT_CODE in commit
    a6e5563dfaff0cb7147058b9c49e38b611a28fb1.
    
    While rhbz#2186531 mentions just libcxx and libcxxabi, libunwind is also
    affected, having static objects built without -fPIC.
    
    Fixes: rhbz#2186531