RTEMS | _Processor_mask_Find_last_set() use breaks GCC14+ builds using "-std" flags (#5501)

[Matteo Concas (@matteo.concas)]

Matteo Concas created an issue: https://gitlab.rtems.org/rtems/rtos/rtems/-/issues/5501



## Summary

`_Processor_mask_Find_last_set()` calls the `flsl()` function. This function is declared in `strings.h` but behind a `__BSD_VISIBLE` guard (which is 0 in our case). This means it is effectively not declared, this is fine most of the time but when using `-std=c11` (or another standard like C99) it will throw an "implicit declaration of function 'flsl' [-Wimplicit-function-declaration]" error.

## Steps to reproduce

Add "-std=c11" to your flags and compile a binary making use of `_Processor_mask_Find_last_set()` will give you this:

```bash
/prefix/bsp/lib/include/rtems/score/processormaskimpl.h: In function '_Processor_mask_Find_last_set':
/prefix/bsp/lib/include/rtems/score/processormaskimpl.h:285:21: error: implicit declaration of function 'flsl' [-Wimplicit-function-declaration]
  285 |   return (uint32_t) __BIT_FLS( CPU_MAXIMUM_PROCESSORS, a );
      |                     ^~~~~~~~~
```

Using `-std=gnu11` instead will "fix" it but we should probably use something else like `__builtin_ctzl()` instead.

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