WeakRef: Set initial value atomically

[jpsamaroo]

Since all accesses to WeakRef from Julia code are atomic, we might as well be atomic when accessing their contents from C (even if it may not matter in practice). Otherwise it's technically possible for another thread to experience a torn read.

[vtjnash]

You need to fix up all the other uses, such as (make tidysrc --output-sync -jN to list all of them):

                    wr->value = (jl_value_t*)jl_nothing;

Note that it should not be technically possible in this particular case, since we haven't published the value yet (so the compiler is free to turn this back into a normal store anyways)

[vtjnash]

Since all accesses to WeakRef from Julia code are atomic.

Note that this is only weakly true from Julia. And this object is otherwise only accessed from GC, which is already atomic in accessing all fields of all objects

julia> Base.isfieldatomic(WeakRef, 1)
false

[jpsamaroo]

Aside from the test failures, which appear unrelated, is this good to go @vtjnash ?