nixos/installer/cd-dvd/iso-image: Allow booting on CHRP machines

[OPNA2608]

...& New World ROM Macs

Must be built on powerpc64-linux, cross is blocked by at least:

• rust-bindgen/rustPlatform.bindgenHook: Missing target triplet (& ABI) fixes from CC wrapper #473862

Needs:

• mesa: Only build OpenCL stuff on platforms where it works, fix build on ppc64 #456839
• linux: Adjust config to allow booting of installer ISO on ppc64 #486572

WIP.

• Under QEMU:
  • pseries,x-vof=off: Both variants start booting, but fail on /dev/root not appearing.
  • mac99: Starts booting configuration, but not much more output after systemd[1]: Started Journal Service..
• nixos/modules/installer/cd-dvd/ppm2osbadgeicon.c is a custom tool I hacked together for customising the image displayed in the graphical boot selector on New World ROM Macs, see image above. The default image from GRUB's CHRP script is the GNU logo. I wanted to differentiate the icon for the installer from the icon of the already-installed Debian system (which uses the GRUB default).
  If this is deemed useful, I can throw the code into a separate repo and package it so we don't need to vendor it. If this customisation is deemed too much, that's also fine.
• Related to the above: I have vendored a flake.ppm (pride flake logo), because the image produced by imagemagick on hardware has a tint added to the entire result that results in wrong colours (converting nix-snowflake.svg makes it look more like the old blue-purple colour scheme). Would like to drop this and instead generate it, but shipping weird colours.
• CHRP entry point (/ppc/bootinfo.txt) is mostly copied from GRUB. Would like to eventually generate both the CHRP bootinfo.txt and the New World ROM Mac BootX file from an attrset with format-specific adjustments made to it (different SGML tags & formats used for the boot menu icon, different tag capitalisation requirements).
• Would like to rip some changes that cause rebuilds on other ISO targets out into an initial PR, then adjust this one so it's 0 rebuilds for all our existing installers.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • powerpc64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.