cmd/internal/obj/riscv: clean up lowerJALR

This cleans up the last of the direct obj.Prog rewriting, removing lowerJALR
and replacing it with correct handling for AJALR during instruction encoding.

Change-Id: Ieea125bde30d4c0edd2d9ed1e50160543aa8f330
Reviewed-on: https://go-review.googlesource.com/c/go/+/249077


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Joel Sing <joel@sing.id.au>