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

[Peter Waller via llvm-commits]

peterwaller-arm wrote:

What I see for baremetal builds is that a target triple of `aarch64-none-elf` (`<arch>-<sys>-<env>`) is getting internally defaulted to search for libraries at `aarch64-none-unknown-elf` (`<arch>-<vendor>-<sys>-<env>`) but I understand this represents a change of the `<sys>` part of the triple from `none` to `unknown`?

Is this intended? One consequence is that my existing cross compiler setup which uses symlinked clang binaries `aarch64-none-elf-clang -> clang` stopped working because it searches for builtins at `/lib/clang/19/lib/aarch64-none-unknown-elf/libclang_rt.builtins.a`.

In response to this change I am currently building compiler-rt with an explicit triple of `aarch64-unknown-none-elf`.

My expectation was that I should be able to continue to use my old `aarch64-none-elf-clang` and the extra part of the triple would be considered when searching for libraries, where they would be found at `lib/aarch64-unknown-none/...`. Please correct me if I'm wrong.

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