[llvm] [SDag][ARM][RISCV] Allow lowering CTPOP into a libcall (PR #101786)

[Nathan Chancellor via llvm-commits]

nathanchance wrote:

Just in case it matters, this broke ARM kernel builds too: https://github.com/ClangBuiltLinux/continuous-integration2/actions/runs/10240673671/job/28328987353

```
ERROR: modpost: "__popcountsi2" [arch/arm/crypto/aes-arm-bs.ko] undefined!
```

I assume this is the same root cause (since the reproducer exhibits the same behavior with `--target=arm-linux-gnueabi` but I can reduce something out if it is not.

https://github.com/llvm/llvm-project/pull/101786