cmd/internal/obj/riscv: regenerate instruction encoding table

cmd/internal/obj/riscv: regenerate instruction encoding table

This CL updates riscv instructions by
https://github.com/riscv/riscv-opcodes

which adds instruction:
	APAUSE

And removes the following unused instructions:
	AFENCEI
	AFMVQX
	AFMVXQ
	AHFENCEGVMA
	AHFENCEVVMA
	ASLLIRV32
	ASRAIRV32
	ASRLIRV32
	AURET

Change-Id: I314570c643af3e6bbc9d2cd471b6b39985bcbdff
Reviewed-on: https://go-review.googlesource.com/c/go/+/409415
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: M Zhuo <mzh@golangcn.org>