ia64: Remove _dl_skip_args usage

Since ad43cac4 the generic code already shuffles the argv/envp/auxv
on the stack to remove the ld.so own arguments and thus _dl_skip_args
is always 0.

The startup code is changed to read the _dl_argc and _dl_argv values,
and envp is calculated from argc and argv.

Checked on ia64-linux-gnu.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>