depend-cleanup.sh: handle 32-bit dependencies having no 64-bit equivalent

For example with the recent removal of ffs.S for 32-bit arm and i386,
the egrep in clean_dep() did not find any files to remove, even if you
added a "clean_dep lib/libc ffs S" line. This is because it will never
find the ffs.S filename in the 64-bit .depend files for libc.

Split the searching and removing of 32-bit dependencies and objects into
a separate part to cope with this. This can be used similarly later on,
for other bitnesses.

MFC after: 3 days