runtime: add Compiler

R=iant, r, gri
CC=golang-dev
https://golang.org/cl/5720073

src/pkg/runtime/compiler.go

+// 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.
+
+package runtime
+
+// Compiler is the name of the compiler toolchain that built the
+// running binary.  Known toolchains are:
+//
+//	gc      The 5g/6g/8g compiler suite at code.google.com/p/go.
+//	gccgo   The gccgo front end, part of the GCC compiler suite.
+//
+const Compiler = "gc"