perf: enable -matomic for ARC

From https://lkml.org/lkml/2015/10/17/74

Change-Id: Ic7d1c8dcc52c15a77372e89e15697aeb0b4a8411
diff --git a/tools/perf/config/Makefile b/tools/perf/config/Makefile
index 5ad0255..768a610 100644
--- a/tools/perf/config/Makefile
+++ b/tools/perf/config/Makefile
@@ -53,6 +53,11 @@
   LIBUNWIND_LIBS = -lunwind -lunwind-aarch64
 endif
 
+# Additional ARCH settings for ARC
+ifeq ($(ARCH),arc)
+  CFLAGS += -matomic
+endif
+
 ifeq ($(NO_PERF_REGS),0)
   $(call detected,CONFIG_PERF_REGS)
 endif