lib/systems: add minimal s390x-linux cross-compile support

Tested basic functionality as:

    $ nix-build --arg crossSystem '{ config = "s390x-unknown-linux-gnu"; }' -A re2c
    $ file ./result/bin/re2c
    $ ./result/bin/re2c: ELF 64-bit MSB executable, IBM S/390, version 1 (SYSV),
    dynamically linked, interpreter ...-gnu-2.33-50/lib/ld64.so.1, for GNU/Linux 2.6.32, not stripped
    $ qemu-s390x ./result/bin/re2c --version
    re2c 2.2

lib/systems/examples.nix

@@ -152,6 +152,10 @@ rec {
     config = "s390-unknown-linux-gnu";
   };
 
+  s390x = {
+    config = "s390x-unknown-linux-gnu";
+  };
+
   arm-embedded = {
     config = "arm-none-eabi";
     libc = "newlib";