[llvm] [Triple] Add libc-less `x32` environment (PR #202795)

[Alex Rønne Petersen via llvm-commits]

alexrp wrote:

We are currently mapping `x32` to `muslx32` in Zig, but I don't like doing this because it implies that there's a libc in the picture which the compiler can exploit knowledge of, e.g. in `TargetLibraryInfo`. It's the same reason that, for all other targets, we map to the `unknown` environment when not linking libc.

https://github.com/llvm/llvm-project/pull/202795