Very many test failures on big-endian PowerPC

The builder kvm-rpm-centos73-ppc64 builder is effectively disabled from running any tests on 10.5, because it aborts very early:

Too many failed: Failed 8/80 tests, 90.00% were successful.

Failing test(s): binlog_encryption.encrypted_slave binlog_encryption.encrypted_master_switch_to_unencrypted binlog_encryption.encrypted_master binlog_encryption.rpl_loadfile binlog_encryption.rpl_switch_stm_row_mixed

This was first observed in the bb-10.5-19708 branch:
http://buildbot.askmonty.org/buildbot/builders/kvm-rpm-centos73-ppc64/builds/10061/steps/mtr/logs/stdio
The previous 10.5 build on that platform completed fine:
http://buildbot.askmonty.org/buildbot/builders/kvm-rpm-centos73-ppc64/builds/10047/
Ever since the problematic push to 10.5, the platform has been broken:
http://buildbot.askmonty.org/buildbot/builders/kvm-rpm-centos73-ppc64/builds/10069

The culprit seems to be MDEV-20477 Merge binlog extended metadata support from the upstream.