[lld] [lld][NFC] Precommit test for ld -r links FatLTO PIC objects (PR #92817)

[Paul Kirth via llvm-commits]

ilovepi wrote:

Seems like there could be some redundancy between the existing run lines and the new tests, but those should probably be separate cleanups. 

I think, perhaps we should use `opt` + `llc` to make the FatLTO objects, rather than using `llvm-objcopy` to create the section. When we need to check the bitcode is the same we can just use `opt` to compile the IR and `cmp` to the output from the extracted section. Unfortunately, that's kind of testing codegen at the same time. I don't see a good way to decouple that w/o running the risk of not testing the FatLTO outputs correctly...

https://github.com/llvm/llvm-project/pull/92817