cmd/internal/obj/riscv: fix up instruction groupings

Some of the instructions were incorrectly grouped - untangle this and
separate the RV64I instructions, which are under separate sections of
the RISC-V specification.

Change-Id: I232962ab4054bf0b4745887506f51e74ea73f73d
Reviewed-on: https://go-review.googlesource.com/c/go/+/194238


Reviewed-by: Cherry Zhang <cherryyz@google.com>