[llvm] Use LLVM_DEFAULT_TARGET_TRIPLE to find target libs (PR #72498)

[Leandro Lupori via llvm-commits]

https://github.com/luporl commented:

Looking at LLVM_DEFAULT_TARGET_TRIPLE as a fallback seems ok to me.
But it would be nice if someone who knows better this CMake file reviewed it.

https://github.com/llvm/llvm-project/pull/72498