ocaml: phase 6 Sys module constants (+5 tests, 414 total)

os_type="SX", word_size=64, max_array_length, max_string_length,
executable_name="ocaml-on-sx", big_endian=false, unix=true,
win32=false, cygwin=false. Constants-only for now — argv/getenv_opt/
command would need host platform integration.

lib/ocaml/runtime.sx

@@ -366,6 +366,18 @@
      let printf fmt = print_string fmt
    end ;;
 
+   module Sys = struct
+     let os_type = \"SX\"
+     let word_size = 64
+     let max_array_length = 4611686018427387903
+     let max_string_length = 4611686018427387903
+     let executable_name = \"ocaml-on-sx\"
+     let big_endian = false
+     let unix = true
+     let win32 = false
+     let cygwin = false
+   end ;;
+
    module Hashtbl = struct
      let create n = _hashtbl_create n
      let add t k v = _hashtbl_add t k v