Make debug build possible on Windows with HIP backend.

[Exile333]

What is it?

Debug build for HIP backend fails on Windows. It is known problem (see ROCm/ROCm#5826) and was previously fixed for Linux build. However, CMakeFiles.txt on Windows treat HIP files as CXX files, therefore one should provide extra treatment for Windows build.

Fix is to explicitly set "-O2" option for HIP source files at Windows debug build.

If this correction is good enough to be accepted, I will also make PRs for ggml and whisper.cpp repos.

Ways to test and reproduce.

I found this issue while trying to build llama.cpp for Strix Halo (architecture gfx1151). Most likely this error occurs not only on Strix Halo and can be reproduced at any RDNA 2/3/4 AMD GPU.

git clone https://github.com/ggml-org/llama.cpp.git
cd llama.cpp
cmake -G Ninja -B build -DGGML_HIP=ON -DGGML_HIP_ROCWMMA_FATTN=OFF -DGGML_OPENMP=OFF -DAMDGPU_TARGETS=gfx1151 -DCMAKE_RC_COMPILER="C:/Program Files (x86)/Windows Kits/10/bin/10.0.26100.0/x64/rc.exe" -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_BUILD_TYPE=Debug -DCURL_LIBRARY=C:/vcpkg_github/vcpkg/installed/x64-windows/lib/libcurl.lib -DCURL_INCLUDE_DIR=C:/vcpkg_github/vcpkg/installed/x64-windows/include
cmake --build build

Additional tests.

Not needed, current ones are good.

[IMbackK]

Just use CMAKE_CXX_FLAGS_DEBUG and get rid of the debug conditional.

EDIT: never mind we dont want the other CXX sources compiled with optimization turned on.

[IMbackK]

Kind of ugly but i think that yeah this is the only way to only compile the hip sources with optimization turned on in this case.