Windows XP SP2 and Windows 2003 do not support SHA2.

Change-Id: Ica5faed040e9ced8b79fe78d512586e0e8788b3f
Reviewed-on: https://go-review.googlesource.com/8167
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>