runtime: use correct psABI SP alignment before calling libc mmap

Fixes #14384.

Change-Id: Ib025cf2d20754b4c2db52f0a8a4717fd303371d6
Reviewed-on: https://go-review.googlesource.com/19660


Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-on: https://go-review.googlesource.com/22037