cmd/internal/obj/riscv: handle call, jmp and branch

Also provide REG_LR to more clearly define the link register.

Based on the riscv-go port.

Updates #27532

Change-Id: I0805f373682f93b3918a01c21d4ef34eb3817c75
Reviewed-on: https://go-review.googlesource.com/c/go/+/204627


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