Makefile: fix "make clean" for old GNU find

findutils v4.1.x does not have the `-exec CMD {} +' syntax. We can just as
easily use the `-exec CMD {} \;' syntax. However, it will launch a lot more
`rm' processes, so we only use it if the first form fails with an error.
This isn't a perfect solution (`find -exec +' can fail for other reasons)
but it works well enough.

This problem manifests itself in RHEL 4, findutils 4.1.20:

  $ make clean
  rm -f /XXX/mtd-utils/*.o /XXX/mtd-utils/ftl_format  ...
  find: missing argument to `-exec'
  make: *** [clean] Error 1

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>