[compiler-rt] [libcxx] CompilerRT: Normalize COMPILER_RT_DEFAULT_TARGET_TRIPLE (PR #89234)

[David Spickett via llvm-commits]

DavidSpickett wrote:

Please expand the commit message to describe your previous confusion with effective triple, and why target triple is ok. Also that this changes some folder names hence the libcxx change.

So that we have an audit trail here, as it's turning out to be a tricky change. 

https://github.com/llvm/llvm-project/pull/89234