Comment 0 for bug 1858798

[Impact]
xenial/linux after the update to 4.4.208 upstream stable fails to build tools/perf on environments that don't have libunwind installed.

https://launchpad.net/~canonical-kernel-team/+archive/ubuntu/ppa/+build/18530346/+files/buildlog_ubuntu-xenial-powerpc.linux_4.4.0-172.201_BUILDING.txt.gz

[...]
builtin-report.c: In function ‘report__setup_sample_type’:
builtin-report.c:289:6: error: ‘dwarf_callchain_users’ undeclared (first use in this function)
  if (dwarf_callchain_users) {
      ^
[...]

This is caused by the following commit from linux-4.4.y:

commit faece3af80729763bb28449fa803d12c962571bd
Author: Jin Yao <email address hidden>
Date: Fri Oct 11 10:21:22 2019 +0800

    perf report: Add warning when libunwind not compiled in

    [ Upstream commit 800d3f561659b5436f8c57e7c26dd1f6928b5615 ]

[Fix]
Revert the mentioned commit. The variable 'dwarf_callchain_users' was introduced by eabad8c6856f "perf unwind: Do not look just at the global callchain_param.record_mode" which as of now hasn't been backported to stable linux-4.4.y.

[Test]
Build the kernel on PPA for powerpc, ppc64el and s390x.

[Regression Potential]
Null. This commit is supposed to only add a warning and is breaking the build for some architectures.