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

[Rainer Orth via llvm-commits]

rorth wrote:

This patch badly broke the [Solaris/sparcv9 buildbot](https://lab.llvm.org/staging/#/builders/12/builds/264) which has 128-bit long double, but no `*int128_t`.  

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