csky: Add support for perf unwind-libdw

csky: Add support for perf unwind-libdw

This patch add support for DWARF register mappings and libdw registers
initialization, which is used by perf callchain analyzing, eg:

perf record --call-graph=dwarf <COMMAND>

Here is 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>
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Arnd Bergmann <arnd@arnd.de>