[clang] [compiler-rt] [llvm] [CMake] Use Clang to infer the target triple (PR #89425)

[Petr Hosek via cfe-commits]

petrhosek wrote:

> > @wzssyqa Is it OK with you if I go ahead and merge this PR? I plan to follow up with further improvements but this is necessary to fix the build of LLVM runtime libraries in the bootstrapping build.
> 
> Is there a filed issue linked to this?

I filed #97876 and left a `TODO` comment.

https://github.com/llvm/llvm-project/pull/89425