[BUG] distclean gets confused by cp -sfR

[Rob Landley]

If you copy a dirty linux dir as a tree of symlinks and then "make distclean" in
the copy, most of the .o files get skipped.

  $ cd linux; make defconfig; make -j $(nproc); cd ..
  $ cp -sfR $PWD/linux boing
  $ cd boing
  $ make distclean
  $ find . -name '*.o' | wc
    892     892   20017

Reproduced in commit gb19e8c684703 on x86-64 (devuan ascii).