docs: update s390x document for sentencepiece

Signed-off-by: Aaron Teo <[email protected]>

Author: taronaeo

docs/build-s390x.md

@@ -94,6 +94,12 @@ All models need to be converted to Big-Endian. You can achieve this in three cas
 
     The model you are trying to convert must be in `safetensors` file format (for example [IBM Granite 3.3 2B](https://huggingface.co/ibm-granite/granite-3.3-2b-instruct)). Make sure you have downloaded the model repository for this case.
 
+    Ensure that you have installed the required packages in advance
+    ```bash
+    pip3 install -r requirements.txt
+    ```
+
+    Convert the `safetensors` model to `GGUF`
     ```bash
     python3 convert_hf_to_gguf.py \
         --outfile model-name-be.f16.gguf \
@@ -116,7 +122,7 @@ All models need to be converted to Big-Endian. You can achieve this in three cas
 
     ![File Type - gguf](https://img.shields.io/badge/File_Type-gguf-fff)
 
-    The model you are trying to convert must be in `gguf` file format (for example [IBM Granite 3.3 2B](https://huggingface.co/ibm-granite/granite-3.3-2b-instruct-GGUF)). Make sure you have downloaded the model file for this case.
+    The model you are trying to convert must be in `gguf` file format (for example [IBM Granite 3.3 2B GGUF](https://huggingface.co/ibm-granite/granite-3.3-2b-instruct-GGUF)). Make sure you have downloaded the model file for this case.
 
     ```bash
     python3 gguf-py/gguf/scripts/gguf_convert_endian.py model-name.f16.gguf BIG
@@ -189,6 +195,21 @@ IBM VXE/VXE2 SIMD acceleration depends on the BLAS implementation. It is strongl
 
     Answer: Please ensure that your GCC compiler is of minimum GCC 15.1.0 version, and have `binutils` updated to the latest version. If this does not fix the problem, kindly open an issue.
 
+4. Failing to install the `sentencepiece` package using GCC 15+
+
+    Answer: The `sentencepiece` team are aware of this as seen in [this issue](https://github.com/google/sentencepiece/issues/1108).
+
+    As a temporary workaround, please run the installation command with the following environment variables.
+
+    ```bash
+    export CXXFLAGS="-include cstdint"
+    ```
+
+    For example,
+    ```bash
+    CXXFLAGS="-include cstdint" pip3 install -r requirements.txt
+    ```
+
 ## Getting Help on IBM Z & LinuxONE
 
 1. **Bugs, Feature Requests**
@@ -244,3 +265,5 @@ IBM VXE/VXE2 SIMD acceleration depends on the BLAS implementation. It is strongl
 -   ✅ - acceleration available
 -   🚫 - acceleration unavailable, will still run using scalar implementation
 -   ❓ - acceleration unknown, please contribute if you can test it yourself
+
+Last Updated by **Aaron Teo ([email protected])** on July 21, 2025.