RTEMS | [i386] Implement _CPU_Spin_wait() in cpu.h for SMP spin-loop throttling (#5647)

[Wayne Thornton (@wmthornton-dev)]

Issue created by Wayne Thornton: https://gitlab.rtems.org/rtems/rtos/rtems/-/work_items/5647



## 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 i386 architecture port needs to mirror the `x86_64` behavior by implementing the pipeline pause instruction in `cpukit/score/cpu/i386/include/rtems/score/cpu.h`.

Suggested Implementation:
This should utilize the standard x86 pause instruction (historically encoded as `rep; nop,` which executes safely as a standard no-op on legacy IA-32 processors prior to the Pentium 4 / SSE2):

```
static inline void _CPU_Spin_wait( void )
{
  __asm__ volatile( "pause" ::: "memory" );
}
#define _CPU_Spin_wait _CPU_Spin_wait
```

## Verification:
Validate that the implementation compiles cleanly on SMP i386 targets 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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View it on GitLab: https://gitlab.rtems.org/rtems/rtos/rtems/-/work_items/5647
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