[llvm] Don't use libcalls for i128<->fp conversion on 32-bit targets (PR #196330)

[Craig Topper via llvm-commits]

topperc wrote:

> > My understanding is that i128 libcalls are missing because i128 was not a supported type in gcc/clang for most 32bit targets, and so they were not really supported. If we are now changing to supporting i128 then wouldn't it make sense to start emitting and using libcalls?
> 
> libgcc compatibility would be concern if we changing to use libcall by default IMO

The bug mentioned `__BitInt(128)`. Does libgcc have separate functions for `__BitInt` from `__int128`?

https://github.com/llvm/llvm-project/pull/196330