cmd/internal/obj/riscv: add environment call/breakpoint and base counter/timer instructions

This implements assembler support for ECALL/EBREAK, along with base
counter/timer instructions.

Based on riscv-go port.

Updates #27532

Change-Id: I690a9fd835eeddee1fe9a5616d2b2f856d3952b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/195918


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>