Document the vDSO and add a reference parser

It turns out that parsing the vDSO is nontrivial if you don't already
have an ELF dynamic loader around.  So document it in Documentation/ABI
and add a reference CC0-licenced parser.

This code is dedicated to Go issue 1933:
http://code.google.com/p/go/issues/detail?id=1933



Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/a315a9514cd71bcf29436cc31e35aada21a5ff21.1310563276.git.luto@mit.edu


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>