For the sake of maximizing backtrace collection performance, build
the libunwind project submodule with "-O2" compiler flag.

Also, build it with the "-g" compiler flag just in case to simplify
debugging.

Last but not least, pass the same archive-maintaining program used by the
main CMake project to the libunwind project submodule to make the build
homogeneous.

Needed for #7207

NO_CHANGELOG=build
NO_DOC=build
NO_TEST=build