[libclc] [libclc] Fix Unix Makefiles race condition in add_libclc_builtin_library (PR #187449)

[Wenju He via cfe-commits]

wenju-he wrote:

thanks @jhuber6, I agree with you.
I can't reproduce the race condition in llvm-project and see race condition again in our downstream today after the fix. So it is somewhere else in our downstream related to dependency. I'll investigate further. Close this PR.

https://github.com/llvm/llvm-project/pull/187449