Update to the latest version of libunwind

This fixes issues with building libunwind as a DLL with SJLJ
exception handling. (We currently don't build it as a DLL though.)

build-libcxx.sh

@@ -27,7 +27,7 @@ fi
 if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBUNWIND" ]; then
     cd libunwind
     [ -z "$SYNC" ] || git fetch
-    git checkout 2ddcf2461daa5d61c543474aed06b12a8b9ad816
+    git checkout 86ab23972978242b6f9e27cebc239f3e8428b1af
     cd ..
 fi
 if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBCXXABI" ]; then