[clang] [Driver,BareMetal] Replace -lclang_rt.builtins{,-$arch}.a with an absolute path (PR #82424)

[Emil J via cfe-commits]

ekliptik wrote:

Hi, this broke our downstream. What is the motivation for this, and how is this intended to be used with compiler-rt locations being set by multilib.yaml? We have something similar to what ARM does for multilibs.yaml but with different paths, and this prevents the driver from knowing anything about a singular path at which it should find the correct compiler-rt. What makes compiler-rt special in a way that other target libraries aren't? Here's how a part of our deliverable looks like:

```
riscv/lib/rv32imafc/ilp32f/
├── except
│   ├── libc.a
│   └── libm.a
├── libclang_rt.builtins-riscv32.a
└── noexcept
    ├── libc.a
    └── libm.a
```

and there are several such `<arch>/<abi>` directories

https://github.com/llvm/llvm-project/pull/82424