tools build: Allow duplicate objects in the object list

It's sometimes useful to specify the object affiliation to multiple
config options like:

  libperf-$(CONFIG_X86) += tsc.o
  libperf-$(CONFIG_AUXTRACE) += tsc.o

while the object itself is linked only once. Adding the support for this
and ignoring duplicate objects in the object list.

Suggested-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20150826130103.GF22670@krava.redhat.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>