cmd/internal/obj/riscv: fix the offset of JALR transformed from JAL

cmd/internal/obj/riscv: fix the offset of JALR transformed from JAL

Currently, the offset of JALR is zero all the time, which is transformed
from JAL with over ±1MB offset. This causes the segment fault for the
wrong address.

Change-Id: I4dcb3eb13bd1ea71e9eb27f07c03ffec376608ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/538135
Run-TryBot: M Zhuo <mzh@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: M Zhuo <mzh@golangcn.org>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>