runtime: improve the annotation of debugCallV2 for arm64

runtime: improve the annotation of debugCallV2 for arm64

This CL improves the annotation documentation of the debugCallV2 function
for arm64.

Change-Id: Icc2b52063cf4fe779071039d6a3bca1951108eb0
Reviewed-on: https://go-review.googlesource.com/c/go/+/402514
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Eric Fang <eric.fang@arm.com>
Reviewed-by: Alessandro Arzilli <alessandro.arzilli@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>