UT_RELAX_CPU(): Use a compiler barrier.

ut_delay(): Remove the dummy global variable ut_always_false.

RB: 11399
Reviewed-by: Jimmy Yang <jimmy.yang@oracle.com>

Backported from MySQL-5.7 - patch https://github.com/mysql/mysql-server/commit/5e3efb03962838a13afbf1579abbb96aee48ad64

Suggestion by Stewart Smith