perf tools: Display build warning if x86 instruction decoder differs from kernel

perf tools has a copy of the x86 instruction decoder used by the kernel.
The expectation is that the copy will be kept more-or-less in-synch with
the kernel version.  Consequently it is helpful to know if there are
differences.

This patch adds a check into the perf tools build so that a diff is done
on the sources, and a warning is printed if they are different.  Note
that the warning is not fatal and the build continues as normal.

The check is done as part of building the instruction decoder, so, like
a compiler warning, it is not seen unless the instruction decoder has to
be re-compiled. e.g.

  $ make -C tools/perf >/dev/null
  $ echo "/* blah */" >> tools/perf/util/intel-pt-decoder/inat_types.h
  $ make -C tools/perf >/dev/null
  Warning: Intel PT: x86 instruction decoder differs from kernel
  $ make -C tools/perf >/dev/null
  $

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Qiaowei Ren <qiaowei.ren@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1441196131-20632-2-git-send-email-adrian.hunter@intel.com


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