runtime: rename os1_linux.go to os_linux.go

Change-Id: I938f61763c3256a876d62aeb54ef8c25cc4fc90e
Reviewed-on: https://go-review.googlesource.com/21567
Reviewed-by: Minux Ma <minux@golang.org>

diff --git a/src/runtime/os1_linux.go b/src/runtime/os_linux.go
similarity index 100%
rename from src/runtime/os1_linux.go
rename to src/runtime/os_linux.go