[lld] [llvm] [LLVM][LTO] Factor out RTLib calls and allow them to be dropped (PR #98512)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> > Likely a combination of `--ffunction-sections` and `-Wl,--gc-sections` will remove the dead functions, but that doesn't stop `lld` from extracting it and forcing it through codegen, which adds over a second of link time as it stands if you include math definitions.
> 
> Ok that would explain it. `-ffunction-sections` and `-Wl,--gc-sections` are both enabled by default for the Wasm target. But if there are link time savings due to less codegen that would still benefit us. Thanks.

Yeah, it'll make a difference if you're trying to do LTO with a static library that contains libcalls, which is a little niche but important for my use-case. I also wonder if we should do this by default for AMDGPU / NVPTX as well, @arsenm WDYT?

https://github.com/llvm/llvm-project/pull/98512