[WIP] Build nixpkgs with gcc 8 by default

[fpletz]

This PR bumps gcc used in stdenv to version 8. The main motivation for this is the ability to add -fstack-clash-protection to our default hardening flags (#53753). This will be done in a separate PR.

I anticipate quite a few of broken packages because there are some new checks, i.e. for array bounds and format truncation.

There is also an old gcc8 branch which I noticed just now thanks to @Synthetica9. I'll cherry-pick changes from there.

Hydra Job: https://hydra.nixos.org/jobset/nixpkgs/gcc8
Newly failing builds with gcc8: https://hydra.nixos.org/eval/1500732?compare=staging-next

Current state: Most packages seem to build. clang segfaults currently. gcc fails on Darwin.

[edolstra]

I see 172 commits, maybe you need to rebase this?

[vcunat]

I believe it's mainly because it got master commits that are missing from staging at this point.

[fpletz]

I've rebased this branch onto staging-next because there are some python packages failing on staging that prevent us to check for gcc8 build failures for lots of packages.

[matthewbauer]

Let's aim for this right after the 19.03 branch off. We still have quite a few things in staging that I am wanting to prioritize.

[matthewbauer]

Sorry didn't mean to close it!

[vcunat]

Why the hell was f959c99 detected as merged to staging? It certainly doesn't seem reachable ATM.

[vcunat]

Well, GitHub still thinks it's merged and thus won't let me reopen in-place. Instead I opened #63132 so we don't lose the work.