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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

@MaskRay Can you comment on the core of this issue, i.e. this:
```console
$ clang --target=armv7-w64-windows-gnu -print-effective-triple
thumbv7-w64-windows-gnu
$ clang --target=armv7-w64-windows-gnu -print-runtime-dir
<prefix>/lib/clang/19/lib/armv7-w64-windows-gnu
```
We could work around this by applying a replacement back from `thumbv*` to `armv*` on the output from `clang -print-effective-triple`, but that's not very pretty...

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