runtime: cleanup after conversion to Go

Change-Id: I7c41cc6a5ab9fb3b0cc3812cf7e9776884658778
Reviewed-on: https://go-review.googlesource.com/4671
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/os_plan9.go

@@ -6,6 +6,11 @@ package runtime
 
 import "unsafe"
 
+func close(fd int32) int32
+
+//go:noescape
+func open(name *byte, mode, perm int32) int32
+
 //go:noescape
 func pread(fd int32, buf unsafe.Pointer, nbytes int32, offset int64) int32