Reduce optimization level while building Cabal

As noted in #16817 (closed), Cabal contributes a great deal to our overall build times. In general we build Cabal twice during a fresh build:

• stage0: Either while building hadrian or ghc-cabal, in the case of the make build system.
• stage2: The Cabal library that will ship with the stage2 compiler.

The performance of the stage0 build does not matter at all, so we can reduce the optimisation level freely.

The stage2 build is slightly trickier. In the interest of keeping this patch non-controversial I have opted to only reduce the optimisation level in the developer build flavours.

However, in general the only time that the Cabal library shipped with ghc is used is when building Setup.hs scripts. It seems unnecessary to spend significant build time (and increased binary distribution size) by shipping an optimized Cabal just for this.