runtime: avoid write barrier in cgo mmap code

runtime: avoid write barrier in cgo mmap code

Tested by hand with a runtime/cgo modified to return an mmap failure
after 10 calls.

This is an interim patch.  For 1.7 we should fix mmap properly to avoid
using the same value as both a pointer and an errno value.

Fixes #14149.

Change-Id: I8f2bbd47d711e283001ba73296f1c34a26c59241
Reviewed-on: https://go-review.googlesource.com/19084
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>