This fixes three cases of missing write barriers as found by wbcheck (see #13213). They're likely all quite rare, but I think all real issues.

• The timew was assigned to Time::tm without a write barrier. This is usually a FIXNUM, but can be rational or bignum for large dates.
• zone was sometimes assigned without a write barrier
• time_init_copy did not include write barriers. I would expect this to usually be run on a brand new object, but that isn't guaranteed