lib.meta: introduce availableOn to check package availability on given platform

[oxalica]

Motivation for this change

Currently, a pattern lib.any (lib.meta.platformMatch stdenv.hostPlatform) <some_pkg>.meta.platforms is used to check whether a optional feature should be enabled by default. But it is incorrect since it doesn't check meta.badPlatforms. This made package like systemd on riscv linux have kexectools enabled, which doesn't support riscv, leading to an failure. (Related #101651)

This PR replace the pattern with a new function availableOnPlatform to check both meta.{platforms,badPlatforms}.

This PR does NOT cause any rebuilds for currently non-broken packages.

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.

[Atemu]

The name is a bit long, availableOn would probably suffice.

[oxalica]

@Atemu

The name is a bit long, availableOn would probably suffice.

Renamed. Also added docs for it.

[r-burns]

This is a big help for unbreaking risc-v, and seems useful to have in general