Bug#18842925 : SET THREAD PRIORITY IN INNODB MUTEX SPINLOOP
Like "pause" instruction for hyper-threading at Intel CPUs,
POWER has special instructions only for hinting priority of hardware-threads.

Approved by Sunny in rb#6256

Backport of the 5.7 fix - https://github.com/mysql/mysql-server/commit/c92102a6ef0f280bfb56e5585fca0d0cdcc34890
(excluded cache line size patch)

Suggestion by Stewart Smith