pmem_phwsync(): The implementation for POWER ISA v3.1 that is
compatible with libpmem.

pmem_fence(): A dummy implementation for older ISA. While such systems
are unlikely to support MAP_SYNC memory mappings, this could be useful
when running tests with memory-mapped /dev/shm/*/ib_logfile0
(the "fake PMEM"), to ensure that mariadb-backup will be able to
read the latest redo log contents.

pmem_persist_init(): Check the availability of POWER ISA v3.1.
Thanks to Daniel Black for suggesting this.