runtime: always declare nanotime in Go

For libgo it's always defined in C.

Updates https://gcc.gnu.org/PR86535

Change-Id: I8d7d5fcb55cee60b91007cec1f3a6e8aae104f10
Reviewed-on: https://go-review.googlesource.com/c/163743
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/libgo/go/runtime/stubs3.go b/libgo/go/runtime/stubs3.go
index d3e331a..d339787 100644
--- a/libgo/go/runtime/stubs3.go
+++ b/libgo/go/runtime/stubs3.go
@@ -2,13 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !plan9
-// +build !windows
-// +build !nacl
-// +build !freebsd
-// +build !darwin
-// +build !aix
-
 package runtime
 
 func nanotime() int64