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

[Nathan Chancellor via llvm-commits]

nathanchance wrote:

For what it's worth, not everything links against the compiler runtime, such as the Linux kernel. This change breaks building the kernel for RISC-V, as noticed by some CI infrastructure that built with the original change (#99752):

```
ERROR: modpost: "__popcountdi2" [fs/ext4/ext4.ko] undefined!
```

https://lore.kernel.org/all/?q=__popcountdi2

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