perf_counter: add sample user-space to Documentation/perf_counter/

Initial version of kerneltop.c and perfstat.c.
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/perf_counter/Makefile

+BINS = kerneltop perfstat
+
+all: $(BINS)
+
+kerneltop: kerneltop.c perfcounters.h
+	cc -O6 -Wall -lrt `pkg-config --cflags --libs glib-2.0` -o $@ $<
+
+perfstat: kerneltop
+	ln -sf kerneltop perfstat
+
+clean:
+	rm $(BINS)