Call entry point of the executable through register x17. This allows it

Call entry point of the executable through register x17.  This allows it
to be a normal C function that starts with "bti c" when BTI contro flow
integretry enforcement is in place.  Add "bti c" to _dl_bind_start().

Remove unused _rtld_tlsdesc() function to avoid having to add "bti c" to it.

ok deraadt@