ggml-cpu : disable GGML_NNPA by default due to instability (#14880)

ggml-cpu : disable GGML_NNPA by default due to instability (#14880)

* docs: update s390x document for sentencepiece

Signed-off-by: Aaron Teo <redacted>
(cherry picked from commit e086c5e3a7ab3463d8e0906efcfa39352db0a48d)

* docs: update huggingface links + reword

Signed-off-by: Aaron Teo <redacted>
(cherry picked from commit 8410b085ea8c46e22be38266147a1e94757ef108)

* ggml-cpu: disable ggml-nnpa compile flag by default

fixes #14877

Signed-off-by: Aaron Teo <redacted>
(cherry picked from commit 412f4c7c88894b8f55846b4719c76892a23cfe09)

* docs: update s390x build docs to reflect nnpa disable

Signed-off-by: Aaron Teo <redacted>
(cherry picked from commit c1eeae1d0c2edc74ab9fbeff2707b0d357cf0b4d)

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Signed-off-by: Aaron Teo <redacted>