[lld] lld: Add initial support for GNU LTO format (PR #157175)

[Fangrui Song via llvm-commits]

================
@@ -0,0 +1,21 @@
+RUN: not ld.lld -plugin %S/Inputs/plugin.so  2>&1 | FileCheck %s
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MaskRay wrote:

The option is `defm plugin: Eq<"plugin", ...` and so we support many syntax variants, but it isn't needed to test every of them.

We just need
```
-plugin %S/Inputs/plugin.so
--plugin=%S/Inputs/plugin.so
```

it's not necessary to also test `cd %S`

https://github.com/llvm/llvm-project/pull/157175