Local.String() returns "Local" on every OS, but windows.
Change windows code to do like others.

Updates #15568

Change-Id: I7a4d2713d940e2a01cff9d7f5cefc89def07546a
Reviewed-on: https://go-review.googlesource.com/23078Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>