[clang] [llvm] [Driver][RISC-V] Support YAML multilib config with selectRISCVMultilib fallback (PR #184973)

[Sam Elliott via cfe-commits]

lenary wrote:

Thinking more deeply about the atomics issue, you could do it with a `-fmultilib-flag` that you add from the `Mappings`, I think.

```
Flags:
- Name: atomics
  Values:
  - Name: no-atomics
  - Name: atomics
  Default: no-atomics # Easier to detect presence of atomics than absence
```

And then when in `Mappings`:

```
- Match: -march=rv.*_a2p1_.*
  Flags:
  - -fmultilib-flag=atomics
```

Then each `Variant` would need to include `-fmultilib-flag=atomics` in `Flags` when they have atomics, and `-fmultilib-flag=no-atomics` when they don't.


https://github.com/llvm/llvm-project/pull/184973