[ld] Allow custom sections to be under PT_GNU_RELRO

[Cary Coutant]

> >That probably also means that we should graduate PT_GNU_RELRO to the
> >gABI level, as PT_RELRO. Or, alternatively, DT_RELRO and DT_RELROSZ
> >entries in the dynamic table. (I kind of prefer the dynamic table over
> >the program header table for things like this, since it's the dynamic
> >linker, not the kernel loader, that needs to know about it.)
>
> I can foresee people's objection to depriving one value 0x6474e552 from
> the PT_LOOS~PT_HIOS range. (See a recent discussion
> https://groups.google.com/forum/#!msg/x86-64-abi/7sr4E6THl3g/zUU2UPHOAQAJ "RFC: Usefulness of SHT_X86_64_UNWIND")

I didn't mean to imply using the existing PT_GNU_RELRO value. Of
course I'd assign a new value in the gABI range. But this is moot, as
it seems the consensus is that RELRO should remain a GNU extension.

-cary