Add OGETG to the list of ignored operations.

We don't instrument the runtime package, but calls to runtime.getg
can appear in other packages, for example, after inlining
runtime.LockOSThread.

Change-Id: I8d6e91f1f3c8fd1302b596bdead42d588c059911
Reviewed-on: https://go-review.googlesource.com/8553Reviewed-by: Dmitry Vyukov <dvyukov@google.com>