Fix "Bad system call" running i686-linux binaries on x86_64-linux

To determine which seccomp filters to install, we were incorrectly
using settings.thisSystem, which doesn't denote the actual system when
--system is used.

Fixes #2791.

src/libutil/util.hh

@@ -30,6 +30,10 @@ struct Sink;
 struct Source;
 
 
+/* The system for which Nix is compiled. */
+extern const std::string nativeSystem;
+
+
 /* Return an environment variable. */
 string getEnv(const string & key, const string & def = "");