RFC: adding Linux vsyscall-disable and similar backwards-incompatibility flags to ELF headers?

[Andy Lutomirski]

Hi all-

Linux has a handful of weird features that are only supported for
backwards compatibility.  The big one is the x86_64 vsyscall page, but
uselib probably belongs on the list, too, and we might end up with
more at some point.

I'd like to add a way that new programs can turn these features off.
In particular, I want the vsyscall page to be completely gone from the
perspective of any new enough program.  This is straightforward if we
add a system call to ask for the vsyscall page to be disabled, but I'm
wondering if we can come up with a non-syscall way to do it.

I think that the ideal behavior would be that anything linked against
a sufficiently new libc would be detected, but I don't see a good way
to do that using existing toolchain features.

Ideas?  We could add a new phdr for this, but then we'd need to play
linker script games, and I'm not sure that could be done in a clean,
extensible way.

--Andy
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