cmd/internal/obj/riscv: add vector instruction encodings

cmd/internal/obj/riscv: add vector instruction encodings

Regenerate the riscv instruction encoding table with the V extension
enabled. Add constants and names for the resulting 375 instructions.

Change-Id: Icce688493aeb1e9880fb76a0618643f57e481273
Reviewed-on: https://go-review.googlesource.com/c/go/+/595403
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: 鹏程汪 <wangpengcheng.pp@bytedance.com>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Reviewed-by: Michael Pratt <mpratt@google.com>