Revert "Revert "ARM: 7587/1: implement optimized percpu variable access""

This reverts commit c20d15705fe9b7ff1186f635bc4de9235be19612.
Revert "Revert "ARM: 7747/1: pcpu: ensure __my_cpu_offset cannot be re-ordered across barrier()""

This reverts commit 63ef67e0cc4c51eb362c93c309ccaed446f6f1ee.