csky: Add support for libdw

csky: Add support for libdw

This patch add support for DWARF register mappings and libdw registers
initialization, which is used by perf callchain analyzing when
--call-graph=dwarf is given.

Here is the elfutils csky backend patch set:

https://sourceware.org/ml/elfutils-devel/2019-q2/msg00007.html

Signed-off-by: Mao Han <han_mao@c-sky.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: linux-arch@vger.kernel.org
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1555860794-10572-1-git-send-email-guoren@kernel.org
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>