Add support for js-unknown-ghcjs

This adds enough logic to nixpkgs to support the `js-unknown-ghcjs` triple.

lib/systems/parse.nix

@@ -106,10 +106,12 @@ rec {
 
     wasm32   = { bits = 32; significantByte = littleEndian; family = "wasm"; };
     wasm64   = { bits = 64; significantByte = littleEndian; family = "wasm"; };
-    
+
     alpha    = { bits = 64; significantByte = littleEndian; family = "alpha"; };
 
     avr      = { bits = 8; family = "avr"; };
+
+    js       = { bits = 32; significantByte = littleEndian; family = "js"; };
   };
 
   ################################################################################
@@ -188,6 +190,7 @@ rec {
     openbsd = { execFormat = elf;     families = { inherit bsd; }; };
     solaris = { execFormat = elf;     families = { }; };
     windows = { execFormat = pe;      families = { }; };
+    ghcjs   = { execFormat = unknown; families = { }; };
   } // { # aliases
     # 'darwin' is the kernel for all of them. We choose macOS by default.
     darwin = kernels.macos;
@@ -299,6 +302,8 @@ rec {
         then { cpu = elemAt l 0; vendor = elemAt l 1;    kernel = elemAt l 2;                }
       else if (elem (elemAt l 2) ["eabi" "eabihf" "elf"])
         then { cpu = elemAt l 0; vendor = "unknown"; kernel = elemAt l 1; abi = elemAt l 2; }
+      else if (elemAt l 2 == "ghcjs")
+        then { cpu = elemAt l 0; vendor = "unknown"; kernel = elemAt l 2; }
       else throw "Target specification with 3 components is ambiguous";
     "4" =    { cpu = elemAt l 0; vendor = elemAt l 1; kernel = elemAt l 2; abi = elemAt l 3; };
   }.${toString (length l)}