There are a number of platforms affected by [1]. It is easy enough to
check in a cross-platform way that bitcasts aren't using f16<->f32
libcalls; thus, add a generic test covering most supported
architectures, with an XFAIL for targets that are currently broken. As
they get fixed, this test will fail and can be updated.
[1]: https://github.com/llvm/llvm-project/issues/97981
314458197e