Rollup merge of #50569 - michaelwoerister:cross-lang-lto-2, r=alexcrichton

Allow for specifying a linker plugin for cross-language LTO

This PR makes the `-Zcross-lang-lto` flag optionally take the path to the `LLVMgold.so` linker plugin. If this path is specified, `rustc` will invoke the linker with the correct arguments (i.e. `-plugin` and various `-plugin-opt`s).

This can be used to ergonomically enable cross-language LTO for Rust programs with C/C++ dependencies:
```
clang -O2 test.c -otest.o -c -flto=thin
llvm-ar -rv libxxx.a test.o
rustc -L. main.rs -Zcross-lang-lto=/usr/lib64/LLVMgold.so -O -Clink-arg=-fuse-ld=gold
```

- Note that in theory this should work with Gold, LLD, and newer versions of binutils' LD but on my current system I could only get it to work with Gold.
- Also note that this will work best if the Clang version and Rust's LLVM version are close enough. Clang 6.0 works well with the current nightly.

r? @alexcrichton

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