[llvm] fix `llvm.fma.f16` double rounding issue when there is no native support (PR #171904)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I think what would be nice one day is for a frontend to have a way to indicate that a full C23 math library is available

This should be implied by the triple. Nothing is stopping you from just adding these functions to RuntimeLibcallsInfo today 

https://github.com/llvm/llvm-project/pull/171904