[libclc] libclc: Use frexp builtins to implement frexp for amdgpu (PR #185637)

[Wenju He via cfe-commits]

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wenju-he wrote:

Always including generic source directory could be problematic if a target decides to implement its own version of .inc file with same name, as the include directories would become confusing.

https://github.com/llvm/llvm-project/pull/185637