runtime: replace union in MHeap with a struct

Unions break precise GC.
Update #5193.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/8368044

src/pkg/runtime/malloc.h

@@ -416,10 +416,10 @@ struct MHeap
 	byte *arena_end;
 
 	// central free lists for small size classes.
-	// the union makes sure that the MCentrals are
+	// the padding makes sure that the MCentrals are
 	// spaced CacheLineSize bytes apart, so that each MCentral.Lock
 	// gets its own cache line.
-	union {
+	struct {
 		MCentral;
 		byte pad[CacheLineSize];
 	} central[NumSizeClasses];

src/pkg/runtime/mprof.goc

@@ -61,6 +61,8 @@ struct Bucket
 	Bucket	*next;	// next in hash list
 	Bucket	*allnext;	// next in list of all mbuckets/bbuckets
 	int32	typ;
+	// Generally unions can break precise GC,
+	// this one is fine because it does not contain pointers.
 	union
 	{
 		struct  // typ == MProf