[compiler-rt] CompilerRT: Normalize COMPILER_RT_DEFAULT_TARGET_TRIPLE (PR #88835)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

Oh, and also for the non-per-target runtime directory config, even if we patch the builtins build to recognize `thumbv7`, this produces `libclang_rt.builtins-thumbv7.a`, while clang looks for `libclang_rt.builtins-arm.a`.

https://github.com/llvm/llvm-project/pull/88835