[release] Use threaded compression with xz

[thieta]

Summary

Use xz -T0 to use your threads

llvm/utils/release/export.sh

export_sources() {
     # know about the LLVM version itself. That's why we also export a the
     # llvm-version*-<YYYYMMDD> and llvm-git*-<YYYYMMDD> files.
     git_rev=$(git rev-parse $tree_id)
     echo "git revision: $git_rev"
     echo "$release" > $target_dir/llvm-release-$yyyymmdd.txt
     echo "$rc" > $target_dir/llvm-rc-$yyyymmdd.txt
     echo "$git_rev" > $target_dir/llvm-git-revision-$yyyymmdd.txt
     
-    git archive --prefix=llvm-project-$release$rc.src/ $tree_id . | xz >$target_dir/$(template_file llvm-project)
+    git archive --prefix=llvm-project-$release$rc.src/ $tree_id . | xz -T0 >$target_dir/$(template_file llvm-project)
     popd
 
     if [ -z "$snapshot" ]; then
         if [ ! -d test-suite-$release$rc.src ]; then
             echo "Fetching LLVM test-suite source ..."
             mkdir -p test-suite-$release$rc.src
             curl -L https://github.com/llvm/test-suite/archive/$tag.tar.gz | \
                 tar -C test-suite-$release$rc.src --strip-components=1 -xzf -
         fi
         echo "Creating tarball for test-suite ..."
         tar --sort=name --owner=0 --group=0 \
             --pax-option=exthdr.name=%d/PaxHeaders/%f,delete=atime,delete=ctime \
             -cJf test-suite-$release$rc.src.tar.xz test-suite-$release$rc.src
     fi
 
     for proj in $projects; do
         echo "Creating tarball for $proj ..."
         pushd $llvm_src_dir/$proj
-        git archive --prefix=$proj-$release$rc.src/ $tree_id . | xz >$target_dir/$(template_file $proj)
+        git archive --prefix=$proj-$release$rc.src/ $tree_id . | xz -T0 >$target_dir/$(template_file $proj)
         popd
     done
 }
 
 while [ $# -gt 0 ]; do
     case $1 in
         -release | --release )
             shift