[AArch64][SME] Add intrinsics for multi-vector BFCLAMP (#88251)

According to the specification in
https://github.com/ARM-software/acle/pull/309 this adds the intrinsics

```
  svbfloat16x2_t svclamp[_single_bf16_x2](svbfloat16x2_t zd, svbfloat16_t zn,
                                        svbfloat16_t zm)  __arm_streaming;

  svbfloat16x4_t svclamp[_single_bf16_x4](svbfloat16x4_t zd, svbfloat16_t zn,
                                        svbfloat16_t zm)  __arm_streaming;
```
These are available only  if __ARM_FEATURE_SME_B16B16 is enabled.