Changing gcc optimization flag from -O0 to -Og

We are compiling with -O0 that disables all compiler optimizations. This was to offer a good debugging experience, in particular to have the core dumps understandable.

However, recently Philipp Brummer made me aware of the existence of -Og flag that enables all -O1 optimization except for those that may interfere with debugging.

I believe we can get some performance boost while keep a reasonable debugging experience.

Details: https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html