release/20.x: [libclc] Include isnan implementation for SPIR-V targets

[karolherbst]

The fma software emulation requires it.

Similar to #124614

This change intentionally targets 20.x as things changed on main enough that it's not feasible to land a change in main first.

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@karolherbst (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix. When you are done, please add the release:note label to this PR.