[compiler-rt] [builtins] Convert more int to fp functions to use common implementation (PR #67540)

[Alexander Richardson via llvm-commits]

arichardson wrote:

This seems to have broken __floatuntitf for x86_64 (noticed after rebasing https://github.com/llvm/llvm-project/pull/68132 on top of this).

https://github.com/llvm/llvm-project/pull/67540