[CMake] Make FP16 relocatable

[BwL1289]

Looks like someone did this a few years ago but it was never merged: https://github.com/Maratyszcza/FP16/pull/13/files.

@Maratyszcza can we get this merged to main?

[BwL1289]

Actually, Connor's fork is preferred #23