stdenv: use gcc9 for aarch64

[FRidh]

Several aarch64 builds fail with is referenced by DSO followed by
bin/ld: final link failed: bad value. This seems related to libgcc_s.so.

#108111

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[FRidh]

@GrahamcOfBorg build stdenv

[FRidh]

@flokli you have access to an aarch64 machine right? Could you test this change a bit?

[FRidh]

Let's see how this turns out.

[flokli]

@flokli you have access to an aarch64 machine right? Could you test this change a bit?

Nah, it's not powerful enough to build huge rebuilds. We could have tested on the community builder, though ;-)

[edolstra]

I think using a different gcc for different architectures is a bad idea, guaranteed to cause lots of confusion. Maybe we should just stick to GCC 9 for now.

[FRidh]

Definitely not ideal, but this needs to be resolved for 21.03. Users/maintainers of other platforms need to step up as well.

#108305