Add builtin likely to assertions

[Zack Weinberg]

On Tue, Jul 28, 2020 at 7:10 PM Aditya K via Libc-alpha
<libc-alpha@sourceware.org> wrote:
> The true part of assertions are more likely to get executed so adding likely attribute
> to the true part will help with better basic block layout.

Do you actually observe code layout changes with this patch, and if
so, with which compiler? This should already be handled by the
heuristic that any code path leading to a noreturn function is
unlikely.

zw