runtime: rename os1_darwin.go to os_darwin.go

Change-Id: If0e0bc5a85101db1e70faaab168fc2d12024eb93
Reviewed-on: https://go-review.googlesource.com/22005
Reviewed-by: Ian Lance Taylor <iant@golang.org>

diff --git a/src/runtime/os1_darwin.go b/src/runtime/os_darwin.go
similarity index 100%
rename from src/runtime/os1_darwin.go
rename to src/runtime/os_darwin.go