libunwind: update to 1.2

Addresses CVE-2015-3239: Off-by-one error in the dwarf_to_unw_regnum
function in include/dwarf_i.h in libunwind 1.1 allows local users to
have unspecified impact via invalid dwarf opcodes.

Upstream stable-v1.2 fixed the missing unwind_i.h issue but no new
tarball is released yet

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>