lib/systems: Support FreeBSD

A tricky thing about FreeBSD is that there is no stable ABI across
versions. That means that putting in the version as part of the config
string is paramount.

We have a parsed represenation that separates name versus version to
accomplish this. We include FreeBSD versions 12 and 13 to demonstrate
how it works.

lib/systems/examples.nix

@@ -303,6 +303,11 @@ rec {
 
   # BSDs
 
+  x86_64-freebsd = {
+    config = "x86_64-unknown-freebsd13";
+    useLLVM = true;
+  };
+
   x86_64-netbsd = {
     config = "x86_64-unknown-netbsd";
   };