Request for review: 7123315 instanceKlass::_static_oop_field_count and instanceKlass::_java_fields_count should be u2 type

[Jiangli Zhou]

The instanceKlass::_static_oop_field_count and 
instanceKlass::_java_fields_count are defined as int type currently. The 
VM spec specifies the 'field_count' as u2. Based on that, these two 
fields can be defined as u2. That would save 4-byte for each loaded 
class when placing the two fields next to each other.

http://cr.openjdk.java.net/~jiangli/7123315/webrev.00/

Tested with runThese -jdk and UTE nsk/sajdi/ tests. Ran jprt. 
Performance testing with specjvm98 and specjbb2005 didn't show any 
performance degradation.

Thanks,

Jiangli