only run stylo_tests's size_of tests on 64-bit platforms

The tests are designed for this, and trying to accommodate 32-bit
platforms is difficult.  Size concerns due to element packing and holes
are less of a problem on 32-bit platforms in any event.

tests/unit/stylo/lib.rs

@@ -13,6 +13,7 @@ extern crate selectors;
 extern crate style_traits;
 
 mod sanity_checks;
+#[cfg(target_pointer_width = "64")]
 mod size_of;
 mod specified_values;
 

tests/unit/stylo/specified_values.rs

@@ -4,7 +4,7 @@
 
 use style;
 
-#[test]
+#[cfg(all(test, target_pointer_width = "64"))]
 fn size_of_specified_values() {
     use std::mem::size_of;
     let threshold = 24;