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>

tools/build/Documentation/Build.txt

@@ -66,6 +66,7 @@ To follow the above example, the user provides following 'Build' files:
   ex/Build:
     ex-y += a.o
     ex-y += b.o
+    ex-y += b.o # duplicates in the lists are allowed
 
     libex-y += c.o
     libex-y += d.o

tools/build/Makefile.build

@@ -63,7 +63,7 @@ quiet_cmd_gen = GEN      $@
 # If there's nothing to link, create empty $@ object.
 quiet_cmd_ld_multi = LD       $@
       cmd_ld_multi = $(if $(strip $(obj-y)),\
-		       $(LD) -r -o $@ $(obj-y),rm -f $@; $(AR) rcs $@)
+		       $(LD) -r -o $@  $(filter $(obj-y),$^),rm -f $@; $(AR) rcs $@)
 
 # Build rules
 $(OUTPUT)%.o: %.c FORCE

tools/build/tests/ex/Build

@@ -1,6 +1,7 @@
 ex-y += ex.o
 ex-y += a.o
 ex-y += b.o
+ex-y += b.o
 ex-y += empty/
 ex-y += empty2/