[llvm] [LLVM] Factor disabled Libcalls into the initializer (PR #98421)

[Artem Belevich via llvm-commits]

Artem-B wrote:

> @Artem-B We probably want to disable most of these for NVPTX as well. Any clue which ones we need to keep around?

I believe we do disable most/all libcalls, but for the life of me I can't find where we're doing that. I vaguely recall it was in some odd place.

https://github.com/llvm/llvm-project/pull/98421