Use O2 on stage1 for faster overall build times with make.

Build times when using the quick flavour:

stage1 opt | time (wall) | time (user) -O1 | 13m | 53m -O2 | 13m | 51m So even when we compile stage2 with -O0 (quick) using -O2 on stage1 is already faster.

The difference is even bigger when freezing stage1 and doing multiple builds or compiling stage2 with optimizations.