cmd/internal/obj/mips: add a flag field to Optab

The flag field will be used for marking unsafe points. This CL
just adds the field, not doing anything with it. The next CL will
make use of it. This is for making the diff simpler.

Change-Id: I6ff5406ba2e53ae8a882184733d88482a2ca8e2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/203938
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>