tools build: Use .s extension for preprocessed assembler code

The "man gcc" says .i extension represents the file is C source code
that should not be preprocessed.  Here, .s should be used.

For clarification,
  .c  ---(preprocess)--->  .i
  .S  ---(preprocess)--->  .s

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Aaro Koskinen <aaro.koskinen@nokia.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Lukas Wunner <lukas@wunner.de>
Link: http://lkml.kernel.org/r/1454263140-19670-1-git-send-email-yamada.masahiro@socionext.com


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