[flang] [llvm] [openmp] Fix Debug Build Using GCC 15 (PR #152223)

[via llvm-commits]

dpalermo wrote:

Don't add this option by default as it will expose backend errors when building for the AMDGPU:

add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-O2>)

https://github.com/llvm/llvm-project/pull/152223