compiler: fix emission of large object layouts

Large object layouts don't fit in a pointer-sized integer and therefore
need to be stored in a global instead. However, the way the data was
stored in these globals was not correct for buffers that don't have
pointers near the end. This commit fixes this issue by using math/big
FillBytes() instead of Bytes().

This gets the unicode package to compile on AVR.

compiler/testdata/gc.go

@@ -20,6 +20,10 @@ var (
 		y [60]uintptr
 		z *byte
 	}
+	struct4 *struct {
+		x *byte
+		y [61]uintptr
+	}
 
 	slice1 []byte
 	slice2 []*int
@@ -50,6 +54,10 @@ func newStruct() {
 		y [60]uintptr
 		z *byte
 	})
+	struct4 = new(struct {
+		x *byte
+		y [61]uintptr
+	})
 }
 
 func newFuncValue() *func() {