runtime: fix 32-bit build

In 32-bit worlds, 8-byte objects are only aligned to 4-byte boundaries.

Change-Id: I91469a9a67b1ee31dd508a4e105c39c815ecde58
Reviewed-on: https://go-review.googlesource.com/2581Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/export_test.go

@@ -117,3 +117,5 @@ func GostringW(w []uint16) (s string) {
 
 var Gostringnocopy = gostringnocopy
 var Maxstring = &maxstring
+
+type Uintreg uintreg

src/runtime/runtime_test.go

@@ -261,8 +261,8 @@ func TestTrailingZero(t *testing.T) {
 		n int64
 		z struct{}
 	}
-	if unsafe.Sizeof(T2{}) != 16 {
-		t.Errorf("sizeof(%#v)==%d, want 16", T2{}, unsafe.Sizeof(T2{}))
+	if unsafe.Sizeof(T2{}) != 8 + unsafe.Sizeof(Uintreg(0)) {
+		t.Errorf("sizeof(%#v)==%d, want %d", T2{}, unsafe.Sizeof(T2{}), 8 + unsafe.Sizeof(Uintreg(0)))
 	}
 	type T3 struct {
 		n byte