[libcxx] Remove clang-18 workaround in picolib build (#133254)

clang-19 changed how Arm triples were normalised and so while we
supported 18 and 19, we could not hard code the path here.

Now that Linaro's bots are running clang-19, and libcxx is going to drop
clang-18 support (https://github.com/llvm/llvm-project/pull/130142) I
have simplified it by hard coding the path again.

I also looked into why this exists in the first place. It was added in
https://reviews.llvm.org/D154246 but not questioned at the time.

It is due to the way we build compiler-rt, which is due to the final
layout we need in the install:
1. The builtins library must be called libclang_rt.builtins.a for clang
to find it. There must not be an architecture name in the filename.
2. That builtins library must be directly in lib/, next to picolib's
installed files.

To achieve number 1 we must set LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON.
However, that causes the file to be installed in a per-target dir which
breaks number 2. So to fix that, we move the builtins library up one
level into lib/.

The alternative is to turn off per-target dirs, which results in a
builtin file with an arch in the name, then rename and move that file
(since it gets installed into lib/generic/).

So in the end, it's the same amount of hacks. I think it's best to keep
the one that uses LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON, as this is the
recommended way to built these days.

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