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

[Petr Hosek via llvm-commits]

petrhosek wrote:

This change only covers compiler-rt but it should apply to all runtimes. I've made a similar change that covers all runtimes in https://reviews.llvm.org/D140925 but I forgot to land before the GitHub migration. I reuploaded that change as https://github.com/llvm/llvm-project/pull/89425 and I intend to land unless there are any objects, and after that we could consider undoing this change.

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