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

[Romaric Jodin via cfe-commits]

rjodinchr wrote:

This is breaking `clspv` as it is completely removing the `clc_sw_fma` implementation (the code is still there, but not reachable).

That implementation is needed for some devices, please do not remove it.

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