[RISCV] Use __extendhfsf2/__truncsfhf2 for fp16 <-> fp32

`__gnu_h2f_ieee` and `__gnu_f2h_ieee` are introduce by ARM and set that as
default name for fp16 and fp32 conversion in LLVM.

However RISC-V GCC using default naming scheme for that, which is
`__extendhfsf2` and `__truncsfhf2` for that, that cause runtime ABI
incompatible issue.

Although we didn't have formal runtime ABI spec to specify those naming
convention yet, but I think it would be great to fix the incompatible
issue first.

And I've plan to create a runtime ABI spec undere psABI spec this year.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D118207