cmd/internal/obj,cmd/asm: add vector registers to riscv64 assembler
This adds V0 through V31 as vector registers, which are available on CPUs
that support the V extension.
Change-Id: Ibffee3f9a2cf1d062638715b3744431d72d451ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/595404
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Mark Ryan <markdryan@rivosinc.com> Reviewed-by: Michael Pratt <mpratt@google.com> Reviewed-by: 鹏程汪 <wangpengcheng.pp@bytedance.com>