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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

Thanks, this form seems to work fine for me, but I second @DavidSpickett 's requests for more elaboration on earlier attempts and what's changed from them (plus a reference to earlier PRs or commits, for review trail).

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