On power, use cpython2 to build pypy3

pypy3.spec

@@ -171,10 +171,16 @@ Patch13: 013-fix-typeerror.patch
 #
 # Turn it off with this boolean, to revert back to rebuilding using CPython
 # and avoid a cycle in the build-time dependency graph:
-
 # Note, pypy3 is built with pypy2, so no dependency cycle
 
+%ifarch %{power64}
+# the build is unreliable with pypy+power, getting random failures
+# so better have a bit slower build than having to build until it pass
+%global use_self_when_building 0
+%else
 %global use_self_when_building 1
+%endif
+
 %if 0%{use_self_when_building}
 # pypy3 can only be build with pypy2
 BuildRequires: pypy2
@@ -821,6 +827,7 @@ CheckPyPy %{name}-stackless
 - Actually call execstack as originally intended
 - Use execstack on all arches (it's available now)
 - Don't ship the debug binaries
+- On power, use cpython2 to build pypy3
 
 * Thu Dec 28 2017 Miro Hrončok <mhroncok@redhat.com> - 5.10.0-2
 - Fixed upstream issues #2717 and #2718 (re-enable test_socket)