runtime: rename os1_windows.go to os_windows.go

Change-Id: I11172f3d0e28f17b812e67a4db9cfe513b8e1974
Reviewed-on: https://go-review.googlesource.com/21565
Reviewed-by: Minux Ma <minux@golang.org>

diff --git a/src/runtime/os1_windows.go b/src/runtime/os_windows.go
similarity index 100%
rename from src/runtime/os1_windows.go
rename to src/runtime/os_windows.go