- Use native memcmp() supplied with C runtime instead of hand-unrolled loop ptr_compare_N loop
Prior to fix  ptr_compare_0()  has  3.7% samples in OLTP-RO in-memory. 
Fix brings this down to 1.8% (all memcmp samples)

- Innodb : fix UT_RELAX_CPU to  be defined  as YieldProcessor, as  was also originally intended 
(but intention was lost in the #ifdef maze

This reduces number of ut_delay() samples in profile from 1.5% to 0.5%