`half` currently uses the default legalization of promoting to a `f32`;
however, this implementation implements math in a way that results in
incorrect rounding. Switch to the soft promote implementation, which
does not have this problem.
The SPARC ABI does not specify a `_Float16` type, so there is no concern
with keeping interface compatibility.
Fixes the SPARC part of
https://github.com/llvm/llvm-project/issues/97975
Fixes the SPARC part of
https://github.com/llvm/llvm-project/issues/97981
549d7c4f35