Unless you get penalised for each line of code you write, that code is fine: succinct and very understandable.
If you do get penalised, just use:
for (var i = 0; i < x; i++) arr.push(x);
on a single line :-)
Beyond that, your best bet would be a function along the lines of:
arr = appendXCopiesOfX (arr, x);
but I don't think you're really gaining anything there since, as mentioned, there should be very little problem understanding such a small loop.
All in all, it's probably a waste of time and effort trying to improve what you currently have.