Extend OptionalObsoleteFiles.inc to delete all lib32 files when MK_LIB32 is
set to no on a system that previously had lib32 libraries installed.
Also, to prevent "make delete-old-dirs" from always deleting lib32 directories
after an installworld, move the lib32 subtree to its own mtree file that only
gets applied when MK_LIB32=yes.
Test: Ran "make delete-old" and "make delete-old-libs" on a system that never
had MK_LIB32 enabled, and on a system where MK_LIB32 was enabled and later
disabled. Did this both on amd64 and powerpc64.
Test: Ran "make tinderbox" without errors.
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D3923
