lib.systems, test.cross.sanity: add test case for #264989

We have several cross-compilation bugs that show up if
hostPlatform!=buildPlatform yet
hostPlatform.config==buildPlatform.config.

These bugs have appeared and disappeared as we've fiddled with the
definition of equality for platform objects.  This commit adds a
clear-cut case where they are *not* equal and never will be, so we
can test it.

lib/systems/examples.nix

@@ -115,6 +115,7 @@ rec {
   };
 
   gnu64 = { config = "x86_64-unknown-linux-gnu"; };
+  gnu64_simplekernel = gnu64 // platforms.pc_simplekernel; # see test/cross/default.nix
   gnu32  = { config = "i686-unknown-linux-gnu"; };
 
   musl64 = { config = "x86_64-unknown-linux-musl"; };

pkgs/test/cross/default.nix

@@ -134,6 +134,7 @@ let
     pkgs.pkgsMusl.stdenv
     pkgs.pkgsLLVM.stdenv
     pkgs.pkgsStatic.bash
+    #pkgs.pkgsCross.gnu64_simplekernel.bash   # https://github.com/NixOS/nixpkgs/issues/264989
     pkgs.pkgsCross.arm-embedded.stdenv
     pkgs.pkgsCross.sheevaplug.stdenv  # for armv5tel
     pkgs.pkgsCross.raspberryPi.stdenv  # for armv6l