Test files created with:
$ for f in Libraries/LibJS/Tests/builtins/Date/Date.prototype.get*js; do
cp $f $(echo $f | sed -e 's/get/getUTC/') ;
done
$ rm Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCTime.js
$ git add Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTC*.js
$ ls Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTC*.js | \
xargs sed -i -e 's/get/getUTC/g'
Year computation has to be based on seconds, not days, in case
t is < 0 but t / __seconds_per_day is 0.
Year computation also has to consider negative timestamps.
With this, days is always positive and <= the number of days in the
year, so base the tm_wday computation directly on the timestamp,
and do it first, before t is modified in the year computation.
In C, % can return a negative number if the left operand is negative,
compensate for that.
Tested via test-js. (Except for tm_wday, since we don't implement
Date.prototype.getUTCDate() yet.)
The constructor with a string argument isn't implemented yet, but
this implements the other variants.
The timestamp constructor doens't handle negative timestamps correctly.
Out-of-bound and invalid arguments aren't handled correctly.
