Add zig passthru

[RossComputerGuy]

Adds the zig passthru stuff so this is more usable compared to zig in nixpkgs.

[mitchellh]

Sorry for the delay. Is this still required? If so, can you rebase? Thanks.

[RossComputerGuy]

This is still useful, I just rebased this PR. Plus I modified meta.platforms to now report more accurately what the platforms are based on what's available.

[vkcku]

This change results in the error:

      error: function 'anonymous lambda' called with unexpected argument 'lib'
       at /nix/store/yjrjf262crysn53sscr28aiyi494jqvk-29nxw96bzvbq1iygh54jh2cxjggx2php-source/pkgs/development/compilers/zig/passthru.nix:1:1:
            1| {
             | ^
            2|   stdenv,

See https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/compilers/zig/passthru.nix.

[RossComputerGuy]

Huh, it loaded fine in nix repl for me.

[vkcku]

Maybe an outdated nixpkgs?? It seems to have been removed in NixOS/nixpkgs@1dfa285#diff-37379ceca82f918f0f7e0c7585a136b2501b01f90b92124944641c5d0a6922ed.

[RossComputerGuy]

I've readded it in #93, I was going to fix it last night but didn't have the time.