runtime: add simple malloc benchmarks

Allocs of size 16 can bypass atomic set of the allocated bit, while allocs of size 8 can not.
Allocs with and w/o type info hit different paths inside of malloc.
Current results on linux/amd64:
BenchmarkMalloc8	50000000	        43.6 ns/op
BenchmarkMalloc16	50000000	        46.7 ns/op
BenchmarkMallocTypeInfo8	50000000	        61.3 ns/op
BenchmarkMallocTypeInfo16	50000000	        63.5 ns/op

R=golang-dev, remyoudompheng, minux.ma, bradfitz, iant
CC=golang-dev
https://golang.org/cl/9090045

src/pkg/runtime/malloc_test.go

+// Copyright 2013 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_test
+
+import (
+	"testing"
+	"unsafe"
+)
+
+var mallocSink uintptr
+
+func BenchmarkMalloc8(b *testing.B) {
+	var x uintptr
+	for i := 0; i < b.N; i++ {
+		p := new(int64)
+		x ^= uintptr(unsafe.Pointer(p))
+	}
+	mallocSink = x
+}
+
+func BenchmarkMalloc16(b *testing.B) {
+	var x uintptr
+	for i := 0; i < b.N; i++ {
+		p := new([2]int64)
+		x ^= uintptr(unsafe.Pointer(p))
+	}
+	mallocSink = x
+}
+
+func BenchmarkMallocTypeInfo8(b *testing.B) {
+	var x uintptr
+	for i := 0; i < b.N; i++ {
+		p := new(struct {
+			p [8 / unsafe.Sizeof(uintptr(0))]*int
+		})
+		x ^= uintptr(unsafe.Pointer(p))
+	}
+	mallocSink = x
+}
+
+func BenchmarkMallocTypeInfo16(b *testing.B) {
+	var x uintptr
+	for i := 0; i < b.N; i++ {
+		p := new(struct {
+			p [16 / unsafe.Sizeof(uintptr(0))]*int
+		})
+		x ^= uintptr(unsafe.Pointer(p))
+	}
+	mallocSink = x
+}