Workaround for bug with caml_sys_argv

[lthls]

There's a hack in the compiler that miscompiles the caml_sys_argv primitive if it's declared as an external directly (it's supposed to be exposed through the "%sys_argv" builtin primitive, not directly). I'm not sure exactly why it never was an issue before, but I believe that without this patch trying to call the caml_sys_argv primitive through the generated Prim module will cause a segmentation fault.