cmd/ld: use 64-bit alignment for large data and bss objects

Check for specific, important misalignment in garbage collector.
Not a complete fix for issue 599 but an important workaround.

Update #599.

R=golang-dev, iant, dvyukov
CC=golang-dev
https://golang.org/cl/6641049

src/cmd/ld/data.c

@@ -831,7 +831,9 @@ dosymtype(void)
 static int32
 alignsymsize(int32 s)
 {
-	if(s >= PtrSize)
+	if(s >= 8)
+		s = rnd(s, 8);
+	else if(s >= PtrSize)
 		s = rnd(s, PtrSize);
 	else if(s > 2)
 		s = rnd(s, 4);
@@ -1054,6 +1056,7 @@ dodata(void)
 		datsize += rnd(s->size, PtrSize);
 	}
 	sect->len = datsize - sect->vaddr;
+	datsize = rnd(datsize, PtrSize);
 
 	/* gcdata */
 	sect = addsection(&segtext, ".gcdata", 04);

src/pkg/runtime/mgc0.c

@@ -892,6 +892,12 @@ runtime·gc(int32 force)
 	M *m1;
 	uint32 i;
 
+	// The atomic operations are not atomic if the uint64s
+	// are not aligned on uint64 boundaries. This has been
+	// a problem in the past.
+	if((((uintptr)&work.empty) & 7) != 0)
+		runtime·throw("runtime: gc work buffer is misaligned");
+
 	// The gc is turned off (via enablegc) until
 	// the bootstrap has completed.
 	// Also, malloc gets called in the guts