[libc] [llvm] [LLVM] Replace use of `LLVM_RUNTIMES_TARGET` with `LLVM_DEFAULT_TARGET_TRIPLE` (PR #136208)

[Petr Hosek via llvm-commits]

petrhosek wrote:

> > The issue is that we started using `LLVM_RUNTIMES_TARGET` in a way that wasn't intended for. When I introduced that variable, the "target" was meant as build target (i.e. Ninja or Make target) and used for purposes such as deriving the subdirectory name for build artifacts. It wasn't supposed to be used as the target triple, we should be still using `LLVM_DEFAULT_TARGET_TRIPLE` for that purpose as is already the case in other runtimes like compiler-rt, libunwind, libc++abi and libc++. Unfortunately, it looks like we already started using `LLVM_RUNTIMES_TARGET` as target triple in `flang-rt` and `libc` leading to the issue you've seen. Instead of trying to copt `LLVM_RUNTIMES_TARGET`, my preference would be to replace `LLVM_RUNTIMES_TARGET` in `flang-rt` and `libc` with `LLVM_DEFAULT_TARGET_TRIPLE` and then rename `LLVM_RUNTIMES_TARGET` to something like `LLVM_RUNTIMES_SUBDIR` to clarify its intended purpose.
> 
> That was my first question on the other PR, but you said to fix it in runtimes. I can do that.

Thanks for doing this cleanup!

https://github.com/llvm/llvm-project/pull/136208