RTEMS | [powerpc] cpukit/score/.../cpu.h: Add _CPU_Spin_wait to powerpc (!1367)

[Joel Sherrill (@joel)]

Joel Sherrill commented on a discussion on cpukit/score/cpu/powerpc/include/rtems/score/cpu.h: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1367#note_155421

 >  /** Type that can store a 32-bit integer or a pointer. */
 >  typedef uintptr_t CPU_Uint32ptr;
 >  
 > +/**
 > + * Pauses the CPU pipeline to prevent aggressive speculative execution
 > + * and reduce thermal load during spin-wait loops.
 > + *
 > + * For PowerPC SMP targets (QorIQ e500mc, e5500, e6500), this issues a Power
 > + * ISA yield instruction ('or 27,27,27'). On e6500 cores with hardware
 > + * multithreading enabled, this temporarily deprioritizes the executing
 > + * thread. On single-threaded or legacy cores, this executes safely as a
 > + * no-operation (nop).
 > + */
 > +static inline void _CPU_Spin_wait( void )
 > +{
 > +  __asm__ volatile ( "or 27, 27, 27" : : : "memory" );

@vijay Which form of the _or_ do you think should be used? [Stack Overflow](https://stackoverflow.com/questions/5425506/equivalent-of-x86-pause-instruction-for-ppc) has a discussion including a set of give _or_ variants from Linux along with an image from a PowerPC manual. [glibc's ppc.h](https://codebrowser.dev/glibc/glibc/sysdeps/powerpc/sys/platform/ppc.h.html#53) has static inline functions for the _or_ forms with comments that appear to match the PowerPC manual.

-- 
View it on GitLab: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1367#note_155421
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