compiler.h: add READ_ONCE/WRITE_ONCE macro

These macros are needed to make sure the compiler does not optimize away
atomic constructs such as "while (!READ_ONCE(foo))" loops that do not
modify foo within the loop

Also move the barrier() define where it belongs while we are here, it is
needed for READ_ONCE/WRITE_ONCE and including ihk/cpu.h here causes
include loops

Change-Id: Ia533a849ed674719ccbc0495be47d22a3c47b8f8

lib/include/ihk/cpu.h

@@ -25,8 +25,6 @@ void cpu_safe_halt(void);
 void cpu_restore_interrupt(unsigned long);
 void cpu_pause(void);
 
-#define barrier()   arch_barrier()
-
 unsigned long cpu_disable_interrupt_save(void);
 
 struct ihk_mc_interrupt_handler {