riscv64: Add Zfh extension support

[rjiejie]

This patch make following simple design rules:

• Construct common encoding interface/macro for frond-end & back-end, see VEX/pub/libvex_riscv_common.h
• Every Independent ISA extension is modular and splited as individual file, that will help to maintain all extensions of RISCV

[rjiejie]

However, it makes me wonder how to best approach merging this work. My thinking is to initially submit upstream for review only the current RV64GC support, make maintainers aware that some new Iops will be needed for RISC-V, and once the port lands, have a separate upstream review for the FP16 support.

That also means that I would merge this FP16 work in this repository on a separate branch for now, or the other way around, create a new branch for only the code that should go upstream in the first batch.

Thanks for all comments @petrpavlu @paulfloyd I agree with your opinion, we should upstream RVGC as base code for RISCV first :)