BSD, Linux, and z/OS enable `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` by
default.
When a compiler-rt library is not found, we currently report an
incorrect filename `libclang_rt.XXX-$arch.a`
```
% /tmp/Debug/bin/clang++ a.cc -fsanitize=address -o a
ld.lld: error: cannot open /tmp/Debug/lib/clang/19/lib/linux/libclang_rt.asan-x86_64.a: No such file or directory
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
```
With this change, we will correctly report:
```
% /tmp/Debug/bin/clang++ a.cc -fsanitize=address -o a
ld.lld: error: cannot open /tmp/Debug/lib/clang/19/lib/x86_64-unknown-linux-gnu/libclang_rt.asan.a: No such file or directory
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
```
Link: https://discourse.llvm.org/t/runtime-directory-fallback/76860
b876596a76