superenv: Use 02 optimization flag for Linux builds

`-Os` produces sometimes bigger binaries on Linux.
Also, llvm built with `-Os` is really slow at runtime for Linux.

Using `-02` aligns us with what Debian does, and as we are compiling most of our stuff with gcc (and not clang), it makes sense to use `-02` on Linux.
`-Os` does probably slightly different things when used on mac with llvm, compared to when it is used with gcc on Linux.