[utils] Use parallel xz when creating the test-suite src package

This was not happening before because it's calling xz via tar
instead of xz directly like the other source packages. This
speeds up the compression a lot.

Author: tru

llvm/utils/release/export.sh

@@ -123,7 +123,7 @@ export_sources() {
                 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 \
+        XZ_OPT="-T0" 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