ld.so: Check for new cache format first and enhance corruption check

Now that ldconfig defaults to the new format (only), check for it
first.  Also apply the corruption check added in commit 2954daf0
("Add more checks for valid ld.so.cache file (bug 18093)") to the
new-format-only case.

Suggested-by: Josh Triplett <josh@joshtriplett.org>