sysgpu: replace deprecated @fence builtin

[joshua-holmes]

Wrench is trying to update Zig, but Zig had an update that introduced a breaking change:
ziglang/zig#21585

Due to this change, I have removed @fence(). I am new to atomics, but I believe the removal of @fence() means that we need to ensure that everything after the atomic write, which happens in @atomicRmw(), actually happens after the write, not before - namely the deinitialization of parent. This can be done by changing the ordering of @atomicRmw() from .release to .acq_rel, which is what I have done.

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