CL 49331 increased windows stack commit size to 2MB by mistake.
Revert that change.

Fixes #22439

Change-Id: I919e549e87da326f4ba45890b4d32f6d7046186f
Reviewed-on: https://go-review.googlesource.com/74490
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>