Makes IntrusiveLinkedListSynchronized mutable to avoid allocation on poptask

[andrebsguedes]

Currently poptask is allocating every time it calls into jl_task_get_next due to StickyWorkqueue (IntrusiveLinkedListSynchronized) being immutable and requiring an allocation to be used as Any on the ccall.

The allocations can be seen in the following snippet:

function work()
    done = 0.0
    l = Threads.SpinLock()
    Threads.@sync for _ in 1:(Threads.nthreads() / 2)
        Threads.@spawn begin
            v = 1.0
            for i in 1:(1<<33)
                v *= 1.0001
                if i % (1 << 13) == 0
                    yield()
                end
            end
            Base.@lock_nofail l done += v
        end
    end
    return done
end

julia> @time work()
 15.758794 seconds (5.03 M allocations: 153.523 MiB, 0.35% gc time)

Which after the change becomes:

julia> @time work()
 15.907513 seconds (67 allocations: 4.719 KiB)

[NHDaly]

Ah, Nick pointed me to the upstream PR, which corresponds to this one: JuliaLang#50802

Thanks