compiler: fix races in link queue

[mlugg]

I messed up atomic orderings on this variable because they changed in a local refactor at some point. We need to always release on the store and acquire on the loads so that a linker thread observing .ready sees the stored MIR.

[mlugg]

GitHub in its infinite wisdom seems to have incinerated the corresponding logs, but: there's a very remote possibility that this caused the x86_64-linux-debug-llvm CI failure in https://github.com/ziglang/zig/runs/44014228031. It would've had to be LLVM reordering stuff (despite Debug mode), since x86 has acqrel semantics for all loads and stores, and we would have needed some very unlucky thread scheduling, but it's... literally the only way I could see that failure happening.

EDIT: nope, that was happening because of a really obvious bug which I somehow missed. Thanks @jacobly0 for picking up on it -- fix pushed to this PR.

[jacobly0]

I did some sleuthing and unincinerated the log:

2025-06-13T01:26:55.1382433Z + stage3-debug/bin/zig build test docs --maxrss 21000000000 -Dlldb=/home/ci/deps/lldb-zig/Debug-e0a42bb34/bin/lldb -fqemu -fwasmtime -Dstatic-llvm -Dskip-freebsd -Dskip-netbsd -Dskip-windows -Dskip-macos -Dtarget=native-native-musl --search-prefix /home/ci/deps/zig+llvm+lld+clang-x86_64-linux-musl-0.15.0-dev.233+7c85dc460 --zig-lib-dir /home/ci/actions-runner1/_work/zig/zig/build-debug-llvm/../lib -Denable-superhtml
2025-06-13T01:32:03.8840387Z test
2025-06-13T01:32:03.8840866Z +- test-cases
2025-06-13T01:32:03.8841375Z    +- run safety.exact division failure - vectors
2025-06-13T01:32:03.8842241Z       +- compile exe safety.exact division failure - vectors Debug native failure
2025-06-13T01:32:03.8843255Z error: thread 929707 panic: reached unreachable code
2025-06-13T01:32:03.8844315Z /home/ci/actions-runner1/_work/zig/zig/lib/std/debug.zig:548:14: 0x5a3c9fd in assert (zig)
2025-06-13T01:32:03.8845291Z     if (!ok) unreachable; // assertion failure
2025-06-13T01:32:03.8845868Z              ^
2025-06-13T01:32:03.8846724Z /home/ci/actions-runner1/_work/zig/zig/src/link/Queue.zig:129:11: 0x62504ae in mirReady (zig)
2025-06-13T01:32:03.8862971Z     assert(mir.status.load(.monotonic) != .pending);
2025-06-13T01:32:03.8863596Z           ^
2025-06-13T01:32:03.8864459Z /home/ci/actions-runner1/_work/zig/zig/src/Zcu/PerThread.zig:4403:38: 0x5eebb1b in runCodegen (zig)
2025-06-13T01:32:03.8865625Z     zcu.comp.link_task_queue.mirReady(zcu.comp, out);
2025-06-13T01:32:03.8866935Z                                      ^
2025-06-13T01:32:03.8867946Z /home/ci/actions-runner1/_work/zig/zig/src/Compilation.zig:5367:18: 0x624dc94 in workerZcuCodegen (zig)
2025-06-13T01:32:03.8869052Z     pt.runCodegen(func_index, &air, out);
2025-06-13T01:32:03.8869606Z                  ^
2025-06-13T01:32:03.8870326Z /home/ci/actions-runner1/_work/zig/zig/lib/std/Thread/Pool.zig:180:50: 0x624ddb2 in runFn (zig)
2025-06-13T01:32:03.8871276Z             @call(.auto, func, .{id.?} ++ closure.arguments);
2025-06-13T01:32:03.8871857Z                                                  ^
2025-06-13T01:32:03.8872890Z /home/ci/actions-runner1/_work/zig/zig/lib/std/Thread/Pool.zig:293:27: 0x61c99f7 in worker (zig)
2025-06-13T01:32:03.8873901Z             runnable.runFn(runnable, id);
2025-06-13T01:32:03.8874426Z                           ^
2025-06-13T01:32:03.8875352Z /home/ci/actions-runner1/_work/zig/zig/lib/std/Thread.zig:510:13: 0x5e78e5d in callFn__anon_196858 (zig)
2025-06-13T01:32:03.8876382Z             @call(.auto, f, args);
2025-06-13T01:32:03.8876873Z             ^
2025-06-13T01:32:03.8877627Z /home/ci/actions-runner1/_work/zig/zig/lib/std/Thread.zig:782:30: 0x5cf99c4 in entryFn (zig)
2025-06-13T01:32:03.8878798Z                 return callFn(f, args_ptr.*);
2025-06-13T01:32:03.8879398Z                              ^
2025-06-13T01:32:03.8880419Z lib/libc/musl/src/thread/pthread_create.c:207:17: 0xeb2af3f in start (lib/libc/musl/src/thread/pthread_create.c)
2025-06-13T01:32:03.8881998Z lib/libc/musl/src/thread/x86_64/clone.s:23:0: 0xeb2c34f in ??? (lib/libc/musl/src/thread/x86_64/clone.s)
2025-06-13T01:32:03.8883175Z Unwind error at address `exe:0xeb2c34f` (error.MissingFDE), trace may be incomplete
2025-06-13T01:32:03.8883876Z 
2025-06-13T01:32:03.8883889Z 
2025-06-13T01:32:03.8884161Z error: the following command terminated unexpectedly:
2025-06-13T01:32:03.9005964Z /home/ci/actions-runner1/_work/zig/zig/build-debug-llvm/stage3-debug/bin/zig build-exe -fno-llvm -fno-lld -ODebug -Mroot=/home/ci/actions-runner1/_work/zig/zig/zig-local-cache/o/d14253cb9361aec75981f588c4887bdc/tmp.zig --cache-dir /home/ci/actions-runner1/_work/zig/zig/zig-local-cache --global-cache-dir /home/ci/actions-runner1/_work/