RFC: Add SHT_GNU_PHDRS

[Cary Coutant]

> * Szabolcs Nagy:
>
> > without containing any sections, however elf says
> > "An object file segment contains one or more sections".
>
> Interesting.  This indeed suggests that a covering section for the
> program header is needed, and a new section type for it makes sense.

I would argue that "one or more" is careless wording. It goes on to
say "... though this fact is transparent to the program header.
Whether the file segment holds one or many sections also is immaterial
to program loading."

If you need a special section type just to ensure that the BFD linker
will include the program headers in a loadable segment, that's a mere
implementation detail, and doesn't need to be part of the gABI or
psABI.

-cary