perf tools: Add info about cross compiling for Android ARM

Without defining ARCH=arm, building perf for Android ARM will fail,
because it needs architecture specific files.

So add related relevant information to the android documentation.

Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Irina Tirdea <irina.tirdea@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1351518066-4791-1-git-send-email-js1304@gmail.com


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