Issue created by Wayne Thornton:
https://gitlab.rtems.org/rtems/rtos/rtems/-/work_items/5642
## Description:
We have introduced a new Super Core architecture hook, `_CPU_Spin_wait(),` to
mitigate aggressive speculative execution penalties, reduce thermal load, and
lower bus contention during SMP spin-wait polling loops. The reference template
is currently documented in `cpukit/score/cpu/no_cpu/include/rtems/score/cpu.h`
and natively implemented for `x86_64` upon merging of rtems/rtos/rtems!1256.
The arm architecture port needs to override the default fallback by
implementing a native inline assembly pipeline throttle in
`cpukit/score/cpu/arm/include/rtems/score/cpu.h`.
## Suggested Implementation:
For ARMv7-A / ARMv8-A targets, this should utilize the architectural `yield`
instruction (or `wfe` where appropriate for spin-locks) paired with a compiler
memory clobber:
```
static inline void _CPU_Spin_wait( void )
{
__asm__ volatile( "yield" ::: "memory" );
}
#define _CPU_Spin_wait _CPU_Spin_wait
```
## Verification:
Validate that the implementation compiles cleanly and passes the standalone SMP
spin-wait test suite: `testsuites/smptests/smpspinwait01`. Once defined, the
preprocessor will automatically override the temporary test-level fallback in
`init.c`.
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