[clang] [compiler-rt] [PGO][Offload] Hide GPU entrypoint on Darwin (PR #132966)

[Ethan Luis McDonough via llvm-commits]

EthanLuisMcDonough wrote:

> It worked before your patch right? Is this what it exposed before then?

In 83e180c, I marked `__llvm_write_custom_profile` as used, which fixed the Darwin code stripping bug but broke the library on embedded systems. In c50d39f, I removed the used attribute, which breaks the Darwin code stripping test. This PR should fix both issues.

https://github.com/llvm/llvm-project/pull/132966