update reference to x86_64 ELF ABI

We got this question from Alexander Kruppa (akruppa, at the mail service run by
Google):
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on

http://refspecs.linuxfoundation.org/LSB_4.0.0/LSB-Core-AMD64/LSB-Core-AMD64/normativerefs.html

under

"System V Application Binary Interface AMD64 Architecture Processor Supplement"

you link to the file

http://refspecs.linux-foundation.org/elf/x86_64-abi-0.95.pdf

In the directory

http://refspecs.linux-foundation.org/elf/

there is also the file

http://refspecs.linux-foundation.org/elf/x86_64-abi-0.98.pdf

Is this a valid, updated version of the document? Is it the most
current released one? Was it not linked for a particular reason?
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When going to the history to answer him, I found we didn't have a very good
answer, at least not that I could find. The 0.98 ABI was checked in back in
2007, in response to bug 1615 and at the same time as the 0.95 ABI. But we've
never updated our references to point to it.

We should consider doing this for LSB 5.0.