Update the speed-up build guide

[ulysses4ever]

Following recent thread on ghc-devs, a better approach for a faster build seems to be:

 ./hadrian/build --flavour=default+no_profiled_libs+omit_pragmas --docs=none --skip-perf --freeze1 -j

This is now added to https://gitlab.haskell.org/ghc/ghc/-/wikis/building/hadrian

Maybe this can be reflected on ghc.dev?

[int-index]

This command seems a bit too long compared to hadrian/build -j --flavour=Quick.

If this is the recommended way to do development, maybe we want --flavour=Dev or something that would be equivalent to --flavour=default+no_profiled_libs+omit_pragmas?

And I don’t think --skip-perf does anything when you’re building, it only matters when testing (definitely a good idea to mention it in the “Testing” section, though).

[ulysses4ever]

I don't have a good feeling what would be more appropriate for ghc.dev. Feel free to close if you think none.

--flavour=Dev needs to be negotiated upstream, which I don't feel like doing right now. I posted a question on ghc-devs on why some/all of it is not a part of Quick, though.