runtime: prevent garbage collection during hashmap insertion

Inserting a key-value pair into a hashmap storing keys or values
indirectly can cause the garbage collector to find the hashmap in
an inconsistent	state.

Fixes #5074.

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/7913043

src/pkg/runtime/gc_test.go

@@ -7,6 +7,7 @@ package runtime_test
 import (
 	"os"
 	"runtime"
+	"runtime/debug"
 	"testing"
 )
 
@@ -82,3 +83,17 @@ func TestGcDeepNesting(t *testing.T) {
 		t.Fail()
 	}
 }
+
+func TestGcHashmapIndirection(t *testing.T) {
+	defer debug.SetGCPercent(debug.SetGCPercent(1))
+	runtime.GC()
+	type T struct {
+		a [256]int
+	}
+	m := make(map[T]T)
+	for i := 0; i < 2000; i++ {
+		var a T
+		a.a[0] = i
+		m[a] = T{}
+	}
+}