libunwind: bump to version 1.3.1

libunwind: bump to version 1.3.1

Libunwind provides a sigreturn stub for x86 in version 1.2 [1].  However
the arch still depends on setcontext() which is unavailable in musl-libc
and which is supposed to be "deprecated everywhere" [2]

 [1] x86 sigreturn unimplemented for some libcs,
     https://github.com/libunwind/libunwind/issues/13
 [2] setcontext deprecated on x86,
     https://github.com/libunwind/libunwind/issues/69

Refs: https://github.com/openwrt/packages/issues/8548#issuecomment-497791552
Reported-by: Rosen Penev <rosenp@gmail.com>
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>