[compiler-rt] Add truncxfhf2 with tests to compiler-rt (PR #120372)

[Rainer Orth via llvm-commits]

rorth wrote:

This patch broke the [Solaris/amd64 buildbot](https://lab.llvm.org/staging/#/builders/120/builds/4677).

https://github.com/llvm/llvm-project/pull/120372