runtime: skip test that intermittently hangs on ppc64le

runtime: skip test that intermittently hangs on ppc64le

A new testcase TestSelectFairness was recently added, and
since then the ppc64le build tests have intermittently failed.

This adds a change to skip this test on ppc64le using
SkipFlaky to help determine if the problem is with the
test or something else with that commit.

Updates #22047

Change-Id: Idfef72ed791c5bd45c42ff180947fea3df280ea7
Reviewed-on: https://go-review.googlesource.com/67631
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>