Make the address space manager use fstat64 when it is available. There

Make the address space manager use fstat64 when it is available. There
are two reasons for this:

    - It can cope with manjor and minor device numbers outside the
      traditional 0-255 range.

    - It returns correct results for x86 binaries on amd64 systems
      where fstat returns uninitialised rubbish in the top 16 bits
      of the device number.

We also make the /proc/self/maps reading code encode device numbers in
the new style to cope with manjor and minor device numbers outside the
traditional 0-255 range.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@5116