[libc] [llvm] [libc] Remove obsolete LIBC_HAS_BUILTIN macro (PR #86554)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

We could probably hack around this by putting this in the server if we want to do this patch. Think you could add this to the patch?
```
#ifndef __has_builtin
#define __has_builtin 0
#endif
```

https://github.com/llvm/llvm-project/pull/86554