ggml-cpu: add ggml_thread_cpu_relax with Zihintpause support (#17784)
* ggml-cpu: add ggml_thread_cpu_relax with Zihintpause support
Signed-off-by: Wang Yang <yangwang@iscas.ac.cn>
* cmake: enable RISC-V zihintpause extension for Spacemit builds
* readme : add ZIHINTPAUSE support for RISC-V
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Signed-off-by: Wang Yang <yangwang@iscas.ac.cn>