Use getdents64 syscall on linux.

getdents has been deprecated since linux 2.4 and newer arches (arm64)
might no longer provide the getdents syscall. Use getdents64 for reading
the /proc/self/fd/ dir so --track-fds=yes works reliable on all arches.
Without this the none/tests/fdleak*vgtest might fail.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14384 a5019735-40e9-0310-863c-91ae7b9d1cf9