Update libunwind to the latest version

This version makes libunwind actually make the SjLj context properly
thread local.

build-libcxx.sh

@@ -49,7 +49,7 @@ fi
 if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBUNWIND" ]; then
     cd libunwind
     [ -z "$SYNC" ] || git fetch
-    git checkout 9defb52f575beff21b646e60e63f72ad1ac7cf54
+    git checkout 90c7a8cc28b3f0718e0726eeec32cc4ebb454c7e
     cd ..
 fi
 if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBCXXABI" ]; then