[libclc] [libclc] Bring back fma for clspv (PR #180693)

[Romaric Jodin via cfe-commits]

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rjodinchr wrote:

This is not a copy of what is in the tree.
The FMA implementation in r600 is almost the same as this one, but it is using ulong for the mantissa, which is correct.
This one is using a vector of 2 uint.
This is very important for clspv as we do not want to have a dependency on 64-bit integers, as it is not supported on some old platforms.

https://github.com/llvm/llvm-project/pull/180693