RFC: Add PT_GNU_PROPERTY to cover .note.gnu.property section

[Cary Coutant]

> > I would strongly recommended to try harder to get consensus here.  I do
> > not want to be a in a situation were we revise ABI again two years from
> > now.
>
> That is my hope.  PT_GNU_PROPERTY is our consensus so far.

As you might expect, I support this new program header. Ideally, I'd
have liked to replace the input SHT_NOTE sections with
SHT_GNU_PROPERTY sections and dispense with all the note section
overhead, but I'll take this as a compromise.

-cary