Support both Float16 ABIs depending on LLVM and platform (#49527)
There are two Float16 ABIs in the wild, one for platforms that have a
defing register and the original one where we used i16.
LLVM 15 follows GCC and uses the new ABI on x86/ARM but not PPC.
Co-authored-by: Gabriel Baraldi <baraldigabriel@gmail.com>
