Support non-x86_64 systems

[thoughtpolice]

For work, we have some non-conventional build targets including ARMv7 (and soon AArch64), as well as bog-standard i686-linux that we deploy NixOS and Nix closures onto. (I also personally have a nice AArch64 device that could run Nix well.) We just got done doing a big deployment to them, and something like this would have been a nice alternative over nix-copy-closure.

However, the parser implicitly models the operating system (maybe better to say Platform, to follow meta.platform in mkDerivation) as an ADT with only X86_64_Linux and X86_64_Darwin targets.

I think this should be extended at the very least, but here's another hair-brained suggestion: just make it a free form string, and warn on an "unusual" platform target?

For example, you could take a lesson from arcanist from Phabricator, where if an 'unusual' build target does not match a list of known ones, you simply suggest it was mistyped. For extra bonus points, you could also calculate the levenshtein distance to the list of known platforms and assume a typo at some threshold. Assuming a theoretical new --platform flag, maybe something like:

$ nix-delegate --platform x86_64-linux ... # works normally
...

$ nix-delegate --platform armv7l-linux ... # works normally
...

$ nix-delegate --platform ppc64le-linux ... # unknown target?
WARNING: platform 'ppc64le-linux' seems unusual (I have not heard of it), but proceeding anyway...
...

$ nix-delegate --platform aaarch64-linux ... # oops, typo
WARNING: platform 'aaarch64-linux' seems unusual; maybe you meant 'aarch64-linux'? Proceeding anyway...

A default list of built-in recognized platforms might look like:

platforms =
  [ "i686-linux"
  , "x86_64-linux"
  , "x86_64-darwin"
  , "armv7l-linux" -- maybe some extras, like 'armv6hf-linux' for raspbian/raspberry pi?
  , "aarch64-linux"
  ]

[thoughtpolice]

I would also of course make nix-delegate output any warnings in LARGE, BOLD letters with an ANSI color code on the platform names, to make it pretty and so the user doesn't miss it.