[libclc] [libclc] Use float sw_fma for spirv*-mesa-mesa3d targets (PR #199626)

[Karol Herbst via cfe-commits]

karolherbst wrote:

I think there is a misunderstanding. We don't need fma implemented as a software fma.

We need an exported function that we can use for devices that do not have native fma.

Inside mesa we have logic to control whether `OpFmaKHR` or the `Fma` OpenCL extended instruction is implemented as a native hardware instruction or emulated in software.

It's fine for the SPIR-V binary to end up with those opcodes. What is the problem is, that the software implementation isn't available inside the SPIR-V anymore.

https://github.com/llvm/llvm-project/pull/199626