dwarf2ctf: don't use O_PATH in rel_abs_file_name().

dwarf2ctf: don't use O_PATH in rel_abs_file_name().

O_PATH was introduced in v2.6.39, but fchdir() on an O_PATH opened
handle came in v3.5.  When building UEK3 on a system with a kernel <
v3.5 dwarf2ctf failed with:

  CTF Cannot return to original directory after relative realpath(): Bad
  file descriptor

Open the directory as a normal file descriptor which is a bit uglier but
allows building on systems with older kernels.

Orabug: 19957565

Cc: Nick Alcock <nick.alcock@oracle.com>
Signed-off-by: Jamie Iles <jamie.iles@oracle.com>
Signed-off-by: Guangyu Sun <guangyu.sun@oracle.com>
Acked-by: Kris Van Hees <kris.van.hees@oracle.com>