Use MOVED_LIBS for libraries moved between /usr/lib and /lib.

Add a MOVED_LIBS variable similar to OLD_LIBS except that MOVED_LIBS
is used for the cases that a library's name doesn't change, but it
just moves between /usr/lib and /lib. This will be used by a future
change to auto-generate lib32 old files entries for which these cases
need to be ignored (a moved library remains in /usr/lib32).

Suggested by: emaste
Reviewed by: emaste
Sponsored by: The University of Cambridge, Google Inc.
Differential Revision: https://reviews.freebsd.org/D33848

(cherry picked from commit 57ba3f00f5e9bb6ea2eff5124077508f7a3f40df)