x86-64: new CET-enabled PLT format proposal

[Florian Weimer]

I do wonder if time is better spent on making symbol binding faster in
general, and eliminate the semantic difference between BIND_NOW and lazy
binding (like musl has done, albeit in an IFUNC-less context).

An example of the current performance issues:

  ld.so has poor performance characteristics when loading large
  quantities of .so files
  <https://sourceware.org/bugzilla/show_bug.cgi?id=27695>

I'm not suggesting we bring back prelink.  There must be other
approaches to make binding go faster.

Thanks,
Florian