[dev.cc] runtime: bring back mgc0.h

This was recorded as an hg mv instead of an hg cp.
For now a C version is needed for the Go compiler.

TBR=r
CC=golang-codereviews
https://golang.org/cl/174020043

src/runtime/mgc0.h

+// Copyright 2012 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.
+
+// Used by cmd/gc.
+
+enum {
+	gcBits = 4,
+	BitsPerPointer = 2,
+	BitsDead = 0,
+	BitsScalar = 1,
+	BitsPointer = 2,
+	BitsMask = 3,
+	PointersPerByte = 8/BitsPerPointer,
+	MaxGCMask = 64,
+	insData = 1,
+	insArray,
+	insArrayEnd,
+	insEnd,
+};