Fix MSVC math library link

[rodrigomatta]

Summary

Fixes the MSVC Windows CMake issue where ggml-base could receive MATH_LIBRARY-NOTFOUND.lib as a linker dependency.

On MSVC, math functions are provided by the Visual Studio CRT, so there is no separate libm/m library to link. The top-level CMake now sets MATH_LIBRARY to an empty
cache value for MSVC before loading ggml, preventing MATH_LIBRARY-NOTFOUND from propagating into generated Visual Studio projects.

This PR also brings staging up to date with main.

Verification

• Configured and built CPU Release successfully:
  • cmake -S . -B build-cpu -DCMAKE_BUILD_TYPE=Release -DS2_AUTO_APPLY_LOCAL_PATCHES=ON -DS2_VULKAN=OFF
  • cmake --build build-cpu --config Release
• Configured and built Vulkan Release successfully:
  • cmake -S . -B build-vulkan -DCMAKE_BUILD_TYPE=Release -DS2_AUTO_APPLY_LOCAL_PATCHES=ON -DS2_VULKAN=ON
  • cmake --build build-vulkan --config Release
• Verified both binaries with s2 --help
• Confirmed no generated MATH_LIBRARY-NOTFOUND / NOTFOUND.lib entries in build-cpu or build-vulkan