perf archive: Don't try to collect files without a build-id

To avoid these error:

   [root@doppio ~]# perf archive
   tar: .build-id/00/00000000000000000000000000000000000000: Cannot stat:
   No such file or directory
   tar: .build-id/00/00000000000000000000000000000000000000: Cannot stat:
   No such file or directory
   tar: .build-id/00/00000000000000000000000000000000000000: Cannot stat:
   No such file or directory
   tar: .build-id/00/00000000000000000000000000000000000000: Cannot stat:
   No such file or directory
   tar: Exiting with failure status due to previous errors
   [root@doppio ~]#

More work is needed to support archiving symtabs for binaries
without a build-id, perhaps creating a perf.data UUID + adding
build-ids for the binaries copied into the cache and then have
this perf.data session UUID be a directory with symlinks to the
by now calculated build-id of the files inside it.

Or just do an extra pass and insert the calculated build-ids in
the perf.data header.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>