luajit: fix cross build for 32 bit architectures

[Cynerd]

Description of changes

The 32bit compiler has to be used when building on 64bit system.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

The implementation is tied to my journey here #212494. The magic is in the usage of pkgsi686Linux.buildPackages instead of just plain pkgsi686Linux.

[wegank]

While this looks good, I wonder if there is a more universal solution that works on other crosses.

[Cynerd]

Honestly, I consider the luajit approach of requiring the bitness of the build host to match the build system pretty stupid. It is also the only package I know about that requires it (..although I have not looked for it. It is just from my practice with maintaining packages for OpenWrt.).

I am also afraid that this can't be easily achieved on other platforms. Only some CPUs have the ability to run 32-bit code alongside 64-bit. It is standard on x86_64 but not necessary on ARM or RISC-V (I think that one of the examples is Apple's chips).

I should have added a comment there that it is a speciality of luajit, not some common approach.

[NickCao]

The note should be preserved, this special casing only works x86_64.

[Cynerd]

Except that this comment is almost only just for x86 because that is not some universal option. The only other applicable (also directly supported by nixpkgs) I found is PowerPC.

Nixpkgs provides, in general, only a single target compilers. That is why gcc_multi exists alongside gcc. That gcc_multi currently also hard depends on i686 and doesn't work on other platforms.

$ nix shell nixpkgs#pkgsCross.aarch64-multiplatform.stdenv.cc -c aarch64-unknown-linux-gnu-gcc -E -march=help -xc /dev/null
...
cc1: note: valid arguments are: armv8-a armv8.1-a armv8.2-a armv8.3-a armv8.4-a armv8.5-a
$ nix shell nixpkgs#pkgsCross.aarch64-multiplatform.stdenv_32bit.cc
error: Multilib aarch64-unknown-linux-gnu-stage-final-gcc-wrapper-9.5.0 not supported for ‘aarch64-linux’
(use '--show-trace' to show detailed location information)

I am going to add TODO at the other place instead of this one where support for other platforms should be required. The issue is that it won't be that easy for ARM for example because not every aarch64 supports for example armv7l.

The best practice here would be to use stdenv_32bit but that is broken as I described in #212494. I will suggest that in the TODO.