cmd/internal/objfile,cmd/objdump: add disassembly support for riscv64

cmd/internal/objfile,cmd/objdump: add disassembly support for riscv64

Add support to `go tool objdump` for disassembling riscv64 binaries.

Revendor to bring in cmd/vendor/golang.org/x/arch/riscv64/riscv64asm,
which provides the actual disassembly implementation.

Fixes #36738

Change-Id: I0f29968509041c0c5698fc2d6910a6a0bea9d3c0
Cq-Include-Trybots: luci.golang.try:gotip-linux-riscv64
Reviewed-on: https://go-review.googlesource.com/c/go/+/622257
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>