Enable threads when building libunwind

The thread local SjLj context pointers are only flagged properly
as thread local when building with threads enabled.

Dockerfile

@@ -257,7 +257,7 @@ RUN cd libunwind && \
             -DCMAKE_RANLIB=$TOOLCHAIN_PREFIX/bin/$RANLIB \
             -DLLVM_NO_OLD_LIBSTDCXX=TRUE \
             -DLIBUNWIND_USE_COMPILER_RT=TRUE \
-            -DLIBUNWIND_ENABLE_THREADS=FALSE \
+            -DLIBUNWIND_ENABLE_THREADS=TRUE \
             -DLIBUNWIND_ENABLE_SHARED=FALSE \
             -DLIBUNWIND_ENABLE_CROSS_UNWINDING=FALSE \
             -DCMAKE_CXX_FLAGS="-fsjlj-exceptions -D__USING_SJLJ_EXCEPTIONS__" \