[libclc] [libclc][amdgpu] Implement native_exp via builtin (PR #133696)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

Just realised I missed the brief - `native_exp2` should use the builtin, `native_exp` should continue to call `native_exp2`.

https://github.com/llvm/llvm-project/pull/133696