systems: fix netbsd triple parsing

binutils expects x86_64-unknown-netbsd<version> (only 3 parts!). Any other combo seems to fail.

Also handle darwin versions similarly.

/cc @Ericson2314

lib/systems/parse.nix

···
 with lib.lists;
 with lib.types;
 with lib.attrsets;
+with lib.strings;
 with (import ./inspect.nix { inherit lib; }).predicates;
 
 let
···
   } // { # aliases
     # 'darwin' is the kernel for all of them. We choose macOS by default.
     darwin = kernels.macos;
-    # TODO(@Ericson2314): Handle these Darwin version suffixes more generally.
-    darwin10 = kernels.macos;
-    darwin14 = kernels.macos;
     watchos = kernels.ios;
     tvos = kernels.ios;
     win32 = kernels.windows;
···
         then { cpu = elemAt l 0;                      kernel = elemAt l 1; abi = elemAt l 2; }
       else if (elemAt l 2 == "mingw32") # autotools breaks on -gnu for window
         then { cpu = elemAt l 0; vendor = elemAt l 1; kernel = "windows";  abi = "gnu"; }
+      else if hasPrefix "netbsd" (elemAt l 2)
+        then { cpu = elemAt l 0; vendor = elemAt l 1;    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)}
···
         else if isDarwin  parsed then vendors.apple
         else if isWindows parsed then vendors.pc
         else                     vendors.unknown;
-      kernel = getKernel args.kernel;
+      kernel = if hasPrefix "darwin" args.kernel      then getKernel "darwin"
+               else if hasPrefix "netbsd" args.kernel then getKernel "netbsd"
+               else                                   getKernel args.kernel;
       abi =
         /**/ if args ? abi       then getAbi args.abi
         else if isLinux   parsed then