Implementing C23 _Float16 in ImportC

[Walter Bright]

_Float16 is new in C23 Appendix H.11-6

https://github.com/dlang/dmd/issues/20516

For the moment, I submitted a PR with a workaround:

https://github.com/dlang/dmd/pull/20699

Amazingly, some years ago I implemented 16 bit floats in D:

https://github.com/DigitalMars/sargon/blob/master/src/sargon/halffloat.d

Would anyone like to put halffloat.d into Druntime and make it our 
implementation of _Float16?