[llvm] [GlobalISel][AArch64] Add libcall lowering for fpowi. (PR #67114)

[David Green via llvm-commits]

davemgreen wrote:

^ Rebased.

I was looking at libcall lowering a little while ago, but the patch I had was currently miscompiling when the return types did not match.

https://github.com/llvm/llvm-project/pull/67114