[dev.cc] runtime: convert operating system support code from C to Go

The conversion was done with an automated tool and then
modified only as necessary to make it compile and run.

[This CL is part of the removal of C code from package runtime.
See golang.org/s/dev.cc for an overview.]

LGTM=r
R=r
CC=austin, dvyukov, golang-codereviews, iant, khr
https://golang.org/cl/174830044
diff --git a/src/runtime/os_linux_amd64.go b/src/runtime/os_linux_amd64.go
new file mode 100644
index 0000000..bd6a78e
--- /dev/null
+++ b/src/runtime/os_linux_amd64.go
@@ -0,0 +1,9 @@
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime
+
+// careful: cputicks is not guaranteed to be monotonic!  In particular, we have
+// noticed drift between cpus on certain os/arch combinations.  See issue 8976.
+func cputicks() int64