onnxruntime

[MLAS] Use C-style casting for power vector instructions (#20957)

### Description
Uses C-style casting for Power vector instructions in
`MlasQuantizeLinearInt4Kernel`.



### Motivation and Context
Vector commands (e.g., vec_xst) need C-style casting to support various
compiler versions.
ONNX Runtime CI pipelines do not build with all compiler versions. The
recent INT4 PR broke the powerpc build for certain compiler versions
because it uses C++-style `static_cast<>`.

See:
https://github.com/microsoft/onnxruntime/pull/20362#discussion_r1630106164

Signed-off-by: adrianlizarraga <adlizarraga@microsoft.com>