[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/os2_linux.go b/src/runtime/os2_linux.go
new file mode 100644
index 0000000..eaa9f0e8
--- /dev/null
+++ b/src/runtime/os2_linux.go
@@ -0,0 +1,23 @@
+// Copyright 2009 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
+
+const (
+	_SS_DISABLE  = 2
+	_NSIG        = 65
+	_SI_USER     = 0
+	_SIG_SETMASK = 2
+	_RLIMIT_AS   = 9
+)
+
+// It's hard to tease out exactly how big a Sigset is, but
+// rt_sigprocmask crashes if we get it wrong, so if binaries
+// are running, this is right.
+type sigset [2]uint32
+
+type rlimit struct {
+	rlim_cur uintptr
+	rlim_max uintptr
+}