[libclc] [libclc] Only use software fma for r600 target (PR #179428)

[Konrad Kleine via cfe-commits]

kwk wrote:

@karolherbst In [libclc: disable use of SPV_KHR_fma extension](https://gitlab.freedesktop.org/karolherbst/mesa-libclc/-/commit/0fdefef847d704b0162cacdbafe01350f4531a61) you remove `--spirv-ext=+SPV_KHR_fma
`. But I don't see this mentioned in [this very PR](https://github.com/llvm/llvm-project/pull/179428) that introduced it. Is there some other discussion I should know about?

https://github.com/llvm/llvm-project/pull/179428