ggml-cpu: Fix gcc 15 ICE on ppc64le (#20083) (llama/20130)

ggml-cpu: Fix gcc 15 ICE on ppc64le (#20083) (llama/20130)

This patch addresses an Internal Compiler Error (Segmentation fault)
observed with gcc 15 by replacing the intrinsic + cast by doing
a cat on the data first and then calling the intrinsic. This bypasses the
buggy compiler path while maintaining identical instruction selection.

Performance Verification:
Assembly analysis on RHEL 9 (GCC 15.1.1) confirms that both the original
code and this fix generate the identical Power10 prefixed load instruction:
    `plxv 40, 2(14)`

This ensures zero performance regression while unblocking builds on
newer toolchains.

Reproduced on:
- Alpine Linux + GCC 15.2.0-r2
- RHEL 9  + GCC 15.1.1 (gcc-toolset-15)

Signed-off-by: Shalini Salomi Bodapati <redacted>