flake: Add support for x86_64-freebsd.

Now that we're using nixpkgs 24.11 it's possible to build cross
for FreeBSD from linux, both in a derivation and a devShell.

Change-Id: I06fa257023397a97cdc72af51170e8523e9f1bfb

doc/manual/src/contributing/hacking.md

@@ -149,6 +149,7 @@ Lix can be built for various platforms, as specified in [`flake.nix`]:
 
 - `x86_64-linux`
 - `x86_64-darwin`
+- `x86_64-freebsd`
 - `i686-linux`
 - `aarch64-linux`
 - `aarch64-darwin`

flake.nix

@@ -92,8 +92,7 @@
         "armv7l-linux"
         "riscv64-linux"
         "aarch64-linux"
-        # FIXME: still broken in 24.05: fails to build rustc(??) due to missing -lstdc++ dep
-        # "x86_64-freebsd"
+        "x86_64-freebsd"
         # FIXME: broken dev shell due to python
         # "x86_64-netbsd"
       ];