ggml-cpu : deduplicate scalar implementations

[xctan]

This PR cleans up redundant fallback implementations in each architecture, which were introduced in the previous refactor PR #13892.

[Copilot]

Pull Request Overview

This PR removes redundant scalar fallback implementations from architecture-specific code that were added in a previous refactor, replacing them with calls to generic implementations. The cleanup eliminates duplicated scalar computation code across different CPU architectures while maintaining the same functionality through centralized generic fallback functions.

Key changes:

• Replaced architecture-specific scalar implementations with calls to generic functions
• Added UNUSED macros to silence compiler warnings for variables no longer used in fallback paths
• Maintained existing optimized vectorized code paths while cleaning up fallback implementations

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
ggml/src/ggml-cpu/arch/x86/repack.cpp	Replaced scalar implementations with generic function calls for quantization and matrix operations
ggml/src/ggml-cpu/arch/x86/quants.c	Removed duplicated scalar fallback code for vector dot product operations
ggml/src/ggml-cpu/arch/wasm/quants.c	Cleaned up scalar implementations in WebAssembly-specific code
ggml/src/ggml-cpu/arch/s390/quants.c	Removed redundant scalar fallbacks in s390 architecture code
ggml/src/ggml-cpu/arch/riscv/repack.cpp	Simplified RISC-V repack operations by using generic implementations
ggml/src/ggml-cpu/arch/riscv/quants.c	Restructured conditional compilation and removed scalar duplications
ggml/src/ggml-cpu/arch/powerpc/quants.c	Eliminated redundant scalar code in PowerPC architecture implementation