perf tools: Move libdw availability check before arch Makefile

Since NO_DWARF is used in arch/$(ARCH)/Makefiles, it should be checked
before including those files.  It was moved by mistake during libelf
dependency removal work by me, sorry.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1348824728-14025-2-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>