Add AVX support check for x86_64 architecture in MpcOptions.cmake

[AshAnand34]

This pull request updates the logic for AVX support in the cmake/MpcOptions.cmake file to ensure compatibility with specific system architectures. The most important change involves adding a conditional check for the processor type when MUJOCO_ENABLE_AVX is enabled.

Changes to AVX support logic:

• cmake/MpcOptions.cmake: Added a conditional check to verify if the processor matches x86_64|AMD64 before including CheckAvxSupport and retrieving AVX compile options. This ensures AVX options are only applied on supported architectures. Additionally, a fallback set(AVX_COMPILE_OPTIONS) is added when MUJOCO_ENABLE_AVX is disabled.