[llvm] [LLVM] Add environment triple for 'llvm' (PR #117218)

[Nick Desaulniers via llvm-commits]

https://github.com/nickdesaulniers approved this pull request.

This looks a lot like when the initial musl support was added in 9768b0ae7328d2151b85c612e6686553206d2ab1.

https://github.com/llvm/llvm-project/pull/117218