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

[Romaric Jodin via cfe-commits]

https://github.com/rjodinchr commented:

Note that non-vulkan spirv was not using that clspv implementation before (this implementation makes sure not to add a dependency on int-64 by using vec2 of int-32).

Otherwise LGTM

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