Use maximum compression when packaging release tarballs.

Summary:
Since a full run of test-release.sh takes many hours (at least on my
poor systems), we might as well spend some extra time compressing the
tarball, in return for a quite a bit of gains for uploading and
downloading it.

As an example, the 10.0.0-rc4 .tar.xz tarball shrinks from 465MiB to
306MiB, about 52% smaller.

Reviewers: hans, tstellar, rovka

Reviewed By: hans

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76192