gcov: fix documentation

Commonly available versions of cp and tar don't work well with special
files created using seq_file.  Mention this problem in the gcov
documentation and update the helper script example to work around these
problems.

Signed-off-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>