Default compilation flags

To continue a discussion on Gitter started by @pmp-p ,

> did you test benefits of -O2 -g for real, because I rarely saw a speed improvement vs -Os -g0 (which give a serious size reduction and better jit in v8 )

In my tests some time ago `-O3` was faster by 10% on Pyodide benchmarks and made the overall download size by 10% larger or so as compared to `-Os`, if I remember correctly.

However there is more an more evidence that `-Os` would be preferable https://github.com/pyodide/pyodide/issues/1541 so I agree that we should probably switch to it.

Regarding `-g` vs `-g0` I have not tried it, it would make stack traces less readable right? We should investigate how much size it would save in Pyodide.