cmd/internal/obj/riscv: store relocation details in obj.Prog.RestArgs

cmd/internal/obj/riscv: store relocation details in obj.Prog.RestArgs

Store the relocation offset and symbol in obj.Prog.RestArgs, rather than
overloading obj.Prog.From and having to deal with invalid offsets
potentially existing when the instruction is encoded.

Change-Id: Iff0d678361677e78b41b887f6eba08cee94fccb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/218197
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>