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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> Likely layering violation.

I thought about that, but because the parts we needed were header only I figured it was legal. It's needed for the default calling conventions, which we could move somewhere else if needed.

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