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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> I assume the next step is to wrap RuntimeLibcalls in a proper API separated from TargetLowering?

Yes, I'll pull it out so it can be used externally.

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