[clang] [CMake] Change GCC_INSTALL_PREFIX from warning to fatal error (PR #85891)

[Petr Hosek via cfe-commits]

petrhosek wrote:

> @petrhosek Is there a way to pass flags only to the runtimes portion of the build within the normal workflow? I know we have `-DRUNTIMES_x86_64-unknown-linux-gnu_CMAKE_CXX_COMPILE_FLAGS=` that might work, but I don't think this is respected if we're using the `default` target.

You can use `RUNTIMES_CMAKE_ARGS`, see for example: https://github.com/llvm/llvm-project/blob/c6a65e4b0c80245d766ae2f2f7305b5371d096f5/clang/cmake/caches/Fuchsia-stage2.cmake#L90

https://github.com/llvm/llvm-project/pull/85891