CDS address sanitizer errors

Building with --enable-asan on AArch64 reports an error in java_lang_Class::serialize_offsets (see attachment).

The value `offsets_computed' is a bool which we cast to a u4* and pass to WriteClosure::do_u4. But sizeof(bool) is implementation defined and on AArch64 (and x86_64 also) it is only one byte. Currently this is harmless because offsets_computed is followed by three bytes of padding before an int, but if someone added another bool global variable immediately after offsets_computed we would get some very obscure bugs.