gold RELRO section detection on Sparc

[David Miller]

Output_segment::is_first_section_relro() iterates over the orders
trying to determine if the first data section other than the TLS
section is RELRO.

This scheme doesn't work on Sparc because the PLT is writable.

On other target cpus the PLT is read-only and thus when the PLT
section is created it is merged with the normal text section, thus
avoiding ORDER_PLT from being non-empty as it will be on Sparc.

Can I just add "i == static_cast<i>ORDER_PLT" to the exceptions there
and update the associated comment?

Alternatively, why don't we just start the loop at ORDER_RELRO_LOCAL?

Thanks.