openarena: Clean up and fix tarball evaluation.

It really doesn't make sense to pass stdenv.system to the builder and
check the value during build process. Also, we're now using
$NIX_CC/nix-support/dynamic-linker instead of pattern matching on
ld-linux*.so.?.

This is part of an attempt to fix nixpkgs tarball evaluation.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>