runtime: fix windows-amd64-2012 build

I forgot that in Go assembly, x+16(SP) is not the same as 16(SP).
The former is the virtual stack pointer (one word below FP on x86)
while the latter is the actual stack pointer.

Change-Id: Ibb7012bb97261949f5e1a0dc70869d9a6f50aa99
Reviewed-on: https://go-review.googlesource.com/c/go/+/176557


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>