os: rename remaining four os1_*.go files to os_*.go

Change-Id: Ice9c234960adc7857c8370b777a0b18e29d59281
Reviewed-on: https://go-review.googlesource.com/22853
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

diff --git a/src/runtime/os1_freebsd.go b/src/runtime/os_freebsd.go
similarity index 100%
rename from src/runtime/os1_freebsd.go
rename to src/runtime/os_freebsd.go

diff --git a/src/runtime/os1_nacl.go b/src/runtime/os_nacl.go
similarity index 100%
rename from src/runtime/os1_nacl.go
rename to src/runtime/os_nacl.go

diff --git a/src/runtime/os1_openbsd.go b/src/runtime/os_openbsd.go
similarity index 100%
rename from src/runtime/os1_openbsd.go
rename to src/runtime/os_openbsd.go

diff --git a/src/runtime/os1_plan9.go b/src/runtime/os_plan9.go
similarity index 100%
rename from src/runtime/os1_plan9.go
rename to src/runtime/os_plan9.go