[release-branch.go1.11] runtime: skip stack barrier copy when there are no pointers

After CL 31455, "go fun(n)" may put "n" to write barrier buffer
when there are no pointers in fun's arguments.

Updates #29565

Change-Id: Icfa42b8759ce8ad9267dcb3859c626feb6fda381
Reviewed-on: https://go-review.googlesource.com/c/155779


Reviewed-by: Keith Randall <khr@golang.org>
(cherry picked from commit 5372257e)
Reviewed-on: https://go-review.googlesource.com/c/156357


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>