cppgc: object alignment is not guaranteed for alignment > 8

[lucacasonato]

Right now our cppgc bindings indisciminantly align all objects to 8 bytes. This is problematic if the actual alignment required by T in cppgc::make_garbage_collected<T> is larger than 8, such as when T contains a u128 (where alignment needs to be 16).

You can experience a related SIGSEGV by changing the tests as follows, and running in release (at least on Linux amd64):

--- a/tests/test_cppgc.rs
+++ b/tests/test_cppgc.rs
@@ -36,6 +36,7 @@ fn cppgc_object_wrap() {
   static DROP_COUNT: AtomicUsize = AtomicUsize::new(0);
 
   struct Wrap {
+    force_align: u128,
     value: v8::TracedReference<v8::Value>,
   }
 
@@ -73,6 +74,7 @@ fn cppgc_object_wrap() {
     assert!(obj.is_api_wrapper());
 
     let wrap = Wrap {
+      force_align: 0,
       value: v8::TracedReference::new(scope, args.get(0)),
     };
     let member = unsafe {