[XLA:CPU] Add FP32<->FP16 conversion routines

[XLA:CPU] Add FP32<->FP16 conversion routines

LLVM generates calls to these functions when lowering some fp16 operations on
certain architectures.  These symbols are defined in compiler-rt but we don't
always link to compiler-rt so these symbols are sometimes absent.

This change adds __gnu_f2h_ieee and __gnu_h2f_ieee as weak symbols.  Making them
weak ensures that we are able to build successfully even when linking to a
compiler-rt that defines these symbols.

PiperOrigin-RevId: 186416684