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

[Artem Belevich via llvm-commits]

Artem-B wrote:

> you're likely confusing the "LLVM-IR" libcalls and the "RTLib" libcalls. I know there's logic in TargetLibraryInfo where we disable all except things like printf.

Yup. Thank you for clarifying it. In that case, all RTlib calls should be disabled for NVPTX, as there's nothing on the GPU side to fulfill them at the moment, at least for the standard CUDA compilation. 

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