build-support/rust/lib: Add toTargetFamily

Taken from https://github.com/kolloch/crate2nix/pull/255/files, it
belongs in Nixpkgs not crate2nix.

I have been using that P.R. for a few months without incident.

pkgs/build-support/rust/lib/default.nix

@@ -15,6 +15,21 @@ rec {
     else if platform.isDarwin then "macos"
     else platform.parsed.kernel.name;
 
+  # https://doc.rust-lang.org/reference/conditional-compilation.html#target_family
+  toTargetFamily = platform:
+    if platform ? rustc.platform.target-family
+    then
+      (
+        # Since https://github.com/rust-lang/rust/pull/84072
+        # `target-family` is a list instead of single value.
+        let
+          f = platform.rustc.platform.target-family;
+        in
+        if builtins.isList f then f else [ f ]
+      )
+    else lib.optional platform.isUnix "unix"
+      ++ lib.optional platform.isWindows "windows";
+
   # Returns the name of the rust target, even if it is custom. Adjustments are
   # because rust has slightly different naming conventions than we do.
   toRustTarget = platform: let