7034513: enable fast accessors and empty methods for ZERO and -Xint
Reviewed-by: kvn, iveresov
The fix for 6385687 unconditionally turned off
UseFastEmptyMethods/UseFastAccessorMethods but they should be on for
ZERO and -Xint mode. It was easier to revert the defaults to on and
turn them off for when the compiler is being used.