Fix thisObject for all builtin function calls

Fix thisObject for all builtin function calls

Even when doing

    var v = Object.prototype.valueOf;
    v();

the thisObject must not be converted to the global object automatically, i.e.
remain null. We previously implemented this lack of conversion for apply() and
call(), but it does in fact apply to all built-in functions.

Consequently we can get rid of callDirect and the virtual maybeAdjustThisObject
function and instead just do the "tweak" with the help of a little boolean.

Change-Id: I93bc37f4c6e896d6dcf169aa74953b0e460312ce
Reviewed-by: Lars Knoll <lars.knoll@digia.com>