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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I suspect long-term, we might want to factor this differently... it's sort of annoying for out-of-tree targets to be forced to modify code outside the target itself.  But it's probably simpler to consider the best way to approach that after everything is unified and pulled out of TargetLowering.  So I'm okay with landing this as-is, and revisiting later.

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