os: create sys_bsd.go

R=rsc
CC=golang-dev
https://golang.org/cl/897042
diff --git a/src/pkg/os/Makefile b/src/pkg/os/Makefile
index b69ac24..ebb1665d 100644
--- a/src/pkg/os/Makefile
+++ b/src/pkg/os/Makefile
@@ -15,24 +15,28 @@
 	path.go\
 	proc.go\
 	stat_$(GOOS).go\
-	sys_$(GOOS).go\
 	time.go\
 	types.go\
 
 GOFILES_freebsd=\
 	file_unix.go\
+	sys_bsd.go\
 
 GOFILES_darwin=\
 	file_unix.go\
+	sys_bsd.go\
 
 GOFILES_linux=\
 	file_unix.go\
+	sys_linux.go\
 
 GOFILES_nacl=\
 	file_unix.go\
+	sys_nacl.go\
 
 GOFILES_mingw=\
 	file_mingw.go\
+	sys_mingw.go\
 
 GOFILES+=$(GOFILES_$(GOOS))